Add statistic option to cloudwatch metricset config

[kaiyan-sheng]

This PR is to add statistic as an optional parameter in cloudwatch metricset configuration. If there is no statistic given, then the default statistic methods will be applied: Average, Sum, Maximum, Minimum and SampleCount.

New cloudwatch configuration looks like:

- module: aws
  period: 300s
  metricsets:
    - cloudwatch
  metrics:
    - namespace: AWS/EC2
      name: ["CPUUtilization", "DiskWriteOps"]
      tags.resource_type_filter: ec2:instance
      statistic: ["Maximum", "Minimum"]
    - namespace: AWS/EBS
      statistic: ["Average"]
    - namespace: AWS/ELB
      tags.resource_type_filter: elasticloadbalancing

How to test it

• make sure in the aws account that you will use for testing has EC2, EBS and ELB created with tags on some of them
• pull this PR and run mage update; mage build; ./metricbeat modules enable aws to build metricbeat and enable aws module
• edit ./modules.d/aws.yml to only keep the part for cloudwatch metricset
• start metricbeat if you are using environment variables for aws credentials: AWS_ACCESS_KEY_ID=<access-key-id> AWS_SECRET_ACCESS_KEY=<secret-access-key> AWS_SESSION_TOKEN=<session-token> AWS_REGION=us-east-1 ./metricbeat -e -c metricbeat.yml -d "publish"
• or add credential_profile_name: test-mb into ./modules.d/aws.yml to use credential profile and starts metricbeat with ./metricbeat -e
• check in Kibana to see if metrics are collected from:
  all EC2 instances with only CPUUtilization.max, DiskWriteOps.max, CPUUtilization.min and DiskWriteOps.min
  all EBS and ELB metrics
  EC2 and ELB with tags
  no tags for EBS
  EBS metrics with only .avg statistic
  ELB metrics with all 5 different statistics

closes #12370
closes #12933

[exekias]

This is looking great! It will need a changelog entry (don't forget to add an entry in Breaking Changes section too)

[sayden]

Overall it looks ok. I'll be nice to add some comments in some parts of the code so that it's easier to understand it in the future

[sorantis]

Nice work!
@kaiyan-sheng the CloudWatch documentation for statistics also mentions pNN.NN - The value of the specified percentile. Is it also included in support?

[kaiyan-sheng]

Nice work!
@kaiyan-sheng the CloudWatch documentation for statistics also mentions pNN.NN - The value of the specified percentile. Is it also included in support?

Thanks! Yep it should support pNN.NN as well!