Cloudwatch Input: Support multiple log groups

[jsirianni]

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It would be nice if Cloudwatch input could take a list of log groups, allowing the user to have a single operator defined.

Describe the solution you'd like

• The solution should be backwards compatible, meaning the parameter LogGroupName can stay the same.
• When given something like /aws/rds/instance/*, Stanza should detect the log groups and build a list
• During the polling cycle, we can spin off a Go routine to handle reading from each log group. There should probably be a max number of Go routines that are spawned, this could be user configurable.

If given a single log group, the list can just contain one entry.

Describe alternatives you've considered

Creating additional operators in the pipeline, but this is difficult when you have 10s of them.

Additional context

The end goal is to support something like