Update default value for Cloudwatch rate limit

[AntoineAugusti]

Follow up of: #2298

New documentation: http://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_limits.html

Rate limit is used for the GetMetricStatistics endpoint.

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)
• README.md updated (if adding a new plugin)

[sparrc]

LGTM, thanks!

please update the changelog

[sparrc]

PS: also please put that documentation link in the SampleConfig

[sparrc]

cc @johnrengelman

[AntoineAugusti]

@sparrc Done that and squashed. thanks!

[johnrengelman]

Lgtm

[phemmer]

If we're saying cloudwatch has a limit of 400 requests per second, do we really want to default to telegraf using all of it? This means that if telegraf has it maxed out, and someone simply goes to the AWS dashboard and tries to pull up cloudwatch metrics, their browser is going to push it over the limit. Ditto if they have any other system which also uses cloudwatch metrics. Should we not choose a safer default, and let people increase it to the max if the default isn't enough?

[johnrengelman]

it's probably time revive this PR #1795 so that telegraf can pull multiple periods of data in 1 request thus saving the API limit.

[sparrc]

@phemmer makes a good point....@AntoineAugusti can you lower the default to ~200?

[AntoineAugusti]

@sparrc Sure, done