Cloudwatch multi result

[tmonk42]

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• [na] README.md updated (if adding a new plugin)

This adds a new config option to the CloudWatch input plugin "cw_interval". This can be used in conjunction with 'period' and 'interval' to collect and return multiple results during each run, which might be useful if you want to reduce your CloudWatch API usage while still retaining high resolution and don't mind the lag time on results ending up in InfluxDB.

Default behavior is unchanged if the new option isn't included.

[andyday]

it would be good to have a unit test for this...

[johnrengelman]

This looks useful as well and would be easy to add a test once #1885 is merged.

[sparrc]

I've merged #1885,

you will need to rebase this change, and please try to add a unit test as #1885 has made this much easier.

[sparrc]

requires rebase and unit tests

[sparrc]

thanks for the contribution and apologies for the initial delay in reviewing, feel free to re-open if you can find time to make the requested changes.