[AWS] Add support for collecting metrics from linked cloudwatch accounts using cross-account monitoring

[tommyers-elastic]

Cloudwatch recently added support for cross-account monitoring, which enables cloudwatch customers to setup a single 'monitoring account' which can access cloudwatch metrics from multiple other 'sharing accounts'.

Currently our AWS cloudwatch metrics integration does not support cross account metrics. Only metrics from the monitoring account itself are included when metric collection is configured.

In order to include metrics from linked sharing accounts, the IncludeLinkedAccounts parameter must be included in our ListMetrics API calls. The result of this is that the response contains metrics for the shared accounts as well as the monitoring account. The owning account is also listed for each returned metric, which will allow us to ensure that cloud.account.id is correct on all reported metrics.

[andresrc]

@tommyers-elastic is the goal of this issue to always add IncludeLinkedAccounts in our API calls or to make it configurable?

[tommyers-elastic]

@andresrc we are testing the effects of having this on for non-cross-account scenarios, but if there are no additional side effects i don't see any problems with keeping it enabled.