feat(aws-logs): extractMetric() returns Metric object (#939)

This makes it convenient to immediately use that Metric object to define
Alarms and Dashboards.

Fixes #850.

packages/@aws-cdk/aws-logs/lib/log-group.ts

@@ -1,3 +1,4 @@
+import cloudwatch = require('@aws-cdk/aws-cloudwatch');
 import cdk = require('@aws-cdk/cdk');
 import { LogStream } from './log-stream';
 import { cloudformation } from './logs.generated';
@@ -96,6 +97,7 @@ export abstract class LogGroupRef extends cdk.Construct {
    * @param jsonField JSON field to extract (example: '$.myfield')
    * @param metricNamespace Namespace to emit the metric under
    * @param metricName Name to emit the metric under
+   * @returns A Metric object representing the extracted metric
    */
   public extractMetric(jsonField: string, metricNamespace: string, metricName: string) {
     new MetricFilter(this, `${metricNamespace}_${metricName}`, {
@@ -105,6 +107,8 @@ export abstract class LogGroupRef extends cdk.Construct {
       filterPattern: FilterPattern.exists(jsonField),
       metricValue: jsonField
     });
+
+    return new cloudwatch.Metric({ metricName, namespace: metricNamespace });
   }
 }
 

packages/@aws-cdk/aws-logs/package.json

@@ -59,6 +59,7 @@
     "pkglint": "^0.12.0"
   },
   "dependencies": {
+    "@aws-cdk/aws-cloudwatch": "^0.12.0",
     "@aws-cdk/aws-iam": "^0.12.0",
     "@aws-cdk/cdk": "^0.12.0"
   },

packages/@aws-cdk/aws-logs/test/test.loggroup.ts

@@ -100,7 +100,7 @@ export = {
     const lg = new LogGroup(stack, 'LogGroup');
 
     // WHEN
-    lg.extractMetric('$.myField', 'MyService', 'Field');
+    const metric = lg.extractMetric('$.myField', 'MyService', 'Field');
 
     // THEN
     expect(stack).to(haveResource('AWS::Logs::MetricFilter', {
@@ -114,6 +114,8 @@ export = {
         }
       ]
     }));
+    test.equal(metric.namespace, 'MyService');
+    test.equal(metric.metricName, 'Field');
 
     test.done();
   }