Allow namespace: "*" for cloudwatch metricset

[kaiyan-sheng]

I tried to run cloudwatch metricset with namespace: "*" and ListMetrics API did what we expected: it returned all metrics exists in that AWS account and was able to use to obtain statistical data.

- module: aws
  period: 300s
  credential_profile_name: elastic-test
  metricsets:
    - cloudwatch
  metrics:
    - namespace: "*"
      statistic: ["Average"]

Here is what cloudwatch metricset collected:

The problem is: statistic and period are defined for all metrics. For example, AWS/Usage metrics should be collected every 1 minute instead of every 5 minutes(data loss in this case). Also some metrics like AWS/Billing, should be collected every 6 hours or 12 hours instead of every 5 minutes(introduced extra cost in this case).

The limitations are documented in doc under cloudwatch metricset.
closes #14965

How to test this

In your AWS account, please make sure the services are from more than one namespace.
Enable aws module in Metricbeat and change configuration file aws.yml to be:

- module: aws
  period: 300s
  credential_profile_name: elastic-test
  metricsets:
    - cloudwatch
  metrics:
    - namespace: "*"
      statistic: ["Average"]

Then start Metricbeat. You should be able to see metrics from all existing services from all different namespaces(with only *.avg metrics).

[mtojek]

LGTM.

Does it mean we can deprecate all other modules built upon CloudWatch?