This PR is to add cloudwatch metricset into aws module for metricbeat. cloudwatch metricset will read target namespace from config file and query cloudwatch API ListMetrics and GetMetricData to get monitoring metrics for that specific namespace.

Currently the config for cloudwatch metric looks like:

  cloudwatch_metrics:
    - namespace: AWS/EC2
       metricname: CPUUtilization
       dimensions:
         - name: InstanceId
           value: i-0686946e22cf9494a
    - namespace: AWS/EC2
       metricname: DiskWriteBytes
       dimensions:
         - name: InstanceId
           value: i-0686946e22cf9494a
    - namespace: AWS/EBS
    - namespace: AWS/ELB

Fetch function starts with checking config to see how many listMetrics entries are there as well as namespaces.

- namespace: AWS/EC2
   metricname: CPUUtilization
   dimensions:
     - name: InstanceId
       value: i-0686946e22cf9494a

This count as one listMetrics entry. readCloudwatchConfig reads in the config and stores each listMetrics entry and namespace if there's no metricname information but only namespace, like

- namespace: AWS/ELB

For listMetricsTotal, this is already good for createEvents to use. But for namespacesTotal, we need to loop through each namespace and each region making GetListMetrics api call to get listMetricsOutput and use that for createEvents function.

In createEvents, there is a variable name identifier, which is the combination of all dimensions in listMetricsOutput. For example,

    - namespace: AWS/RDS
      metricname: CommitThroughput
      dimensions:
        - name: DBClusterIdentifier
          value: test1-cluster
        - name: Role
          value: READER

the identifier name is: DBClusterIdentifier,Role
the identifier value is: test1-cluster,READER

Some entries of listMetricsOuput does not have dimensions and that's why there is a eventsNoIdentifier to store events that came from listMetricsOutput that doesn't have dimensions/identifiers.

Please see more details in #11734