Cherry-pick #14971 to 7.x: Allow namespace: "*" for cloudwatch metricset

[kaiyan-sheng]

Cherry-pick of PR #14971 to 7.x branch. Original message:

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