docs(logger): extract Logger code snippets in separate files

docs/snippets/logger/appendKeys.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+// Add persistent log keys via the constructor
+const logger = new Logger({
+    persistentLogAttributes: { 
+        aws_account_id: '123456789012',
+        aws_region: 'eu-west-1',
+        logger: {
+            name: '@aws-lambda-powertools/logger',
+            version: '0.0.1',
+        },
+        extra_key: "some-value"
+    }
+});
+
+// OR add persistent log keys to an existing Logger instance with the appendKeys method:
+// logger.appendKeys({
+//     aws_account_id: '123456789012',
+//     aws_region: 'eu-west-1',
+//     logger: {
+//         name: '@aws-lambda-powertools/logger',
+//         version: '0.0.1',
+//     },
+//     extra_key: "some-value"
+// });    
+
+export const handler = async (_event: any, _context: any): Promise<unknown> => {
+
+    // If you don't want to log the "extra_key" attribute in your logs, you can remove it
+    logger.removeKeys(["extra_key"])
+
+    // This info log will print all extra custom attributes added above
+    // Extra attributes: logger object with name and version of the logger library, awsAccountId, awsRegion
+    logger.info('This is an INFO log');
+    logger.info('This is another INFO log');
+
+    return {
+        foo: 'bar'
+    };
+
+};

docs/snippets/logger/basicUsage.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+const logger = new Logger({ serviceName: 'serverlessAirline' });
+
+export const handler = async (_event, _context): Promise<void> => {
+    // ...
+};

docs/snippets/logger/bringYourOwnFormatterClass.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+import { MyCompanyLogFormatter } from './utils/formatters/MyCompanyLogFormatter';
+
+const logger = new Logger({
+    logFormatter: new MyCompanyLogFormatter(),
+    logLevel: 'DEBUG',
+    serviceName: 'serverlessAirline',
+    sampleRateValue: 0.5,
+    persistentLogAttributes: {
+        awsAccountId: process.env.AWS_ACCOUNT_ID,
+        logger: {
+            name: '@aws-lambda-powertools/logger',
+            version: '0.0.1'
+        }
+    },
+});
+
+export const handler = async (event, context): Promise<void> => {
+
+    logger.addContext(context);
+
+    logger.info('This is an INFO log', { correlationIds: { myCustomCorrelationId: 'foo-bar-baz' } });
+
+};

docs/snippets/logger/bringYourOwnFormatterHandler.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+import { MyCompanyLogFormatter } from './utils/formatters/MyCompanyLogFormatter';
+
+const logger = new Logger({
+    logFormatter: new MyCompanyLogFormatter(),
+    logLevel: 'DEBUG',
+    serviceName: 'serverlessAirline',
+    sampleRateValue: 0.5,
+    persistentLogAttributes: {
+        awsAccountId: process.env.AWS_ACCOUNT_ID,
+        logger: {
+            name: '@aws-lambda-powertools/logger',
+            version: '0.0.1'
+        }
+    },
+});
+
+export const handler = async (event, context): Promise<void> => {
+
+    logger.addContext(context);
+
+    logger.info('This is an INFO log', { correlationIds: { myCustomCorrelationId: 'foo-bar-baz' } });
+
+};

docs/snippets/logger/clearStateDecorator.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+import { LambdaInterface } from '@aws-lambda-powertools/commons';
+
+// Persistent attributes added outside the handler will be 
+// cached across invocations
+const logger = new Logger({
+    logLevel: 'DEBUG',
+    persistentLogAttributes: {
+        foo: "bar",
+        biz: "baz"
+    }
+});
+
+class Lambda implements LambdaInterface {
+    // Enable the clear state flag
+    @logger.injectLambdaContext({ clearState: true })
+    public async handler(_event: any, _context: any): Promise<void> {
+        // Persistent attributes added inside the handler will NOT be cached
+        // across invocations
+        if (event['special_key'] === '123456'){
+            logger.appendKeys({
+                details: { special_key: '123456' }
+            });
+        }
+        logger.debug('This is a DEBUG log');
+    }
+
+}
+
+const myFunction = new Lambda();
+export const handler = myFunction.handler.bind(myFunction); // (1)

docs/snippets/logger/clearStateMiddy.ts

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+import { Logger, injectLambdaContext } from '@aws-lambda-powertools/logger';
+import middy from '@middy/core';
+
+// Persistent attributes added outside the handler will be 
+// cached across invocations
+const logger = new Logger({
+    logLevel: 'DEBUG',
+    persistentLogAttributes: {
+        foo: "bar",
+        biz: "baz"
+    }
+});
+
+const lambdaHandler = async (event: { special_key: string }, _context: any): Promise<void> => {
+    // Persistent attributes added inside the handler will NOT be cached
+    // across invocations
+    if (event['special_key'] === '123456') {
+        logger.appendKeys({
+            details: { special_key: event['special_key'] }
+        });
+    }
+    logger.debug('This is a DEBUG log');
+};
+
+// Enable the clear state flag
+export const handler = middy(lambdaHandler)
+    .use(injectLambdaContext(logger, { clearState: true }));

docs/snippets/logger/createChild.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+// With this logger, all the INFO logs will be printed
+const logger = new Logger({
+    logLevel: 'INFO'
+});
+
+// With this logger, only the ERROR logs will be printed
+const childLogger = logger.createChild({
+    logLevel: 'ERROR'
+});
+
+export const handler = async (_event: any, _context: any): Promise<void> => {
+
+    logger.info('This is an INFO log, from the parent logger');
+    logger.error('This is an ERROR log, from the parent logger');
+
+    childLogger.info('This is an INFO log, from the child logger');
+    childLogger.error('This is an ERROR log, from the child logger');
+
+};

docs/snippets/logger/decorator.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+import { LambdaInterface } from '@aws-lambda-powertools/commons';
+
+const logger = new Logger();
+
+class Lambda implements LambdaInterface {
+    // Decorate your handler class method
+    @logger.injectLambdaContext()
+    public async handler(_event: any, _context: any): Promise<void> {
+        logger.info('This is an INFO log with some context');
+    }
+
+}
+
+const myFunction = new Lambda();
+export const handler = myFunction.handler.bind(myFunction); // (1)

docs/snippets/logger/eventDecorator.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+import { LambdaInterface } from '@aws-lambda-powertools/commons';
+
+const logger = new Logger();
+
+class Lambda implements LambdaInterface {
+    // Set the log event flag to true
+    @logger.injectLambdaContext({ logEvent: true })
+    public async handler(_event: any, _context: any): Promise<void> {
+        logger.info('This is an INFO log with some context');
+    }
+
+}
+
+const myFunction = new Lambda();
+export const handler = myFunction.handler.bind(myFunction); // (1)

docs/snippets/logger/eventMiddy.ts

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+import { Logger, injectLambdaContext } from '@aws-lambda-powertools/logger';
+import middy from '@middy/core';
+
+const logger = new Logger();
+
+const lambdaHandler = async (_event: any, _context: any): Promise<void> => {
+    logger.info('This is an INFO log with some context');
+};
+
+export const handler = middy(lambdaHandler)
+    .use(injectLambdaContext(logger, { logEvent: true }));

docs/snippets/logger/extraData.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+const logger = new Logger();
+
+export const handler = async (event: any, _context: any): Promise<unknown> => {
+
+    const myImportantVariable = {
+        foo: 'bar'
+    };
+
+    // Log additional data in single log items
+
+    // As second parameter
+    logger.info('This is a log with an extra variable', { data: myImportantVariable });
+
+    // You can also pass multiple parameters containing arbitrary objects
+    logger.info('This is a log with 3 extra objects',
+        { data: myImportantVariable },
+        { correlationIds: { myCustomCorrelationId: 'foo-bar-baz' } },
+        { lambdaEvent: event }
+    );
+
+    // Simply pass a string for logging additional data
+    logger.info('This is a log with additional string value', 'string value');
+
+    // Directly passing an object containing both the message and the additional info
+    const logObject = {
+        message: 'This is a log message',
+        additionalValue: 42
+    };
+
+    logger.info(logObject);
+
+    return {
+        foo: 'bar'
+    };
+
+};

docs/snippets/logger/logError.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+const logger = new Logger();
+
+export const handler = async (_event: any, _context: any): Promise<void> => {
+
+    try {
+        throw new Error('Unexpected error #1');
+    } catch (error) {
+        // Log information about the error using the default "error" key
+        logger.error('This is the first error', error as Error);
+    }
+
+    try {
+        throw new Error('Unexpected error #2');
+    } catch (error) {
+        // Log information about the error using a custom "myCustomErrorKey" key
+        logger.error('This is the second error', { myCustomErrorKey: error as Error } );
+    }
+
+};

docs/snippets/logger/logSampling.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+// Notice the log level set to 'ERROR'
+const logger = new Logger({
+    logLevel: 'ERROR',
+    sampleRateValue: 0.5
+});
+
+export const handler = async (_event: any, _context: any): Promise<void> => {
+
+    // This log item (equal to log level 'ERROR') will be printed to standard output
+    // in all Lambda invocations
+    logger.error('This is an ERROR log');
+
+    // These log items (below the log level 'ERROR') have ~50% chance 
+    // of being printed in a Lambda invocation
+    logger.debug('This is a DEBUG log that has 50% chance of being printed');
+    logger.info('This is an INFO log that has 50% chance of being printed');
+    logger.warn('This is a WARN log that has 50% chance of being printed');
+
+    // Optional: refresh sample rate calculation on runtime
+    // logger.refreshSampleRateCalculation();
+
+};

docs/snippets/logger/manual.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+const logger = new Logger();
+
+export const handler = async (_event, context): Promise<void> => {
+
+    logger.addContext(context);
+
+    logger.info('This is an INFO log with some context');
+
+};

docs/snippets/logger/middy.ts

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+import { Logger, injectLambdaContext } from '@aws-lambda-powertools/logger';
+import middy from '@middy/core';
+
+const logger = new Logger();
+
+const lambdaHandler = async (_event: any, _context: any): Promise<void> => {
+    logger.info('This is an INFO log with some context');
+};
+
+export const handler = middy(lambdaHandler)
+    .use(injectLambdaContext(logger));

docs/snippets/logger/sam.ts

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+import { Logger } from '@aws-lambda-powertools/logger';
+
+// Logger parameters fetched from the environment variables (see template.yaml tab)
+const logger = new Logger();
+
+// You can also pass the parameters in the constructor
+// const logger = new Logger({
+//     logLevel: 'WARN',
+//     serviceName: 'serverlessAirline'
+// });

docs/snippets/logger/unitTesting.ts

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+const dummyContext = {
+    callbackWaitsForEmptyEventLoop: true,
+    functionVersion: '$LATEST',
+    functionName: 'foo-bar-function',
+    memoryLimitInMB: '128',
+    logGroupName: '/aws/lambda/foo-bar-function',
+    logStreamName: '2021/03/09/[$LATEST]abcdef123456abcdef123456abcdef123456',
+    invokedFunctionArn: 'arn:aws:lambda:eu-west-1:123456789012:function:foo-bar-function',
+    awsRequestId: 'c6af9ac6-7b61-11e6-9a41-93e812345678',
+    getRemainingTimeInMillis: () => 1234,
+    done: () => console.log('Done!'),
+    fail: () => console.log('Failed!'),
+    succeed: () => console.log('Succeeded!'),
+};
+
+describe('MyUnitTest', () => {
+
+    test('Lambda invoked successfully', async () => {
+
+        const testEvent = { test: 'test' };
+        await handler(testEvent, dummyContext);
+
+    });
+
+});