Check AWS ListMetrics API call without giving namespace

[kaiyan-sheng]

For cloudwatch metricset, check what happens if namespace is not given when making ListMetrics API call. If we can make ListMetrics API call without giving namespace, then we can make cloudwatch metricset to collect metrics for all available resources.

[kaiyan-sheng]

@exekias @sorantis 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).

[kaiyan-sheng]

Very small change in the code to enable this: #14971

[sorantis]

Thanks for checking this @kaiyan-sheng. I think it won't hurt to add this wildcard, as long as we make it explicit in the documentation that the chosen period has certain implications.

[kaiyan-sheng]

@sorantis Sounds good, I will update the PR!