change jolokia input to use bulk requests

[phemmer]

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)

This adjusts the jolokia input to use bulk requests. This significantly increases the performance of it.
In my specific use case, I have a deployment where it takes ~10.4 seconds to gather all the requested jolokia metrics. With this change it now takes ~0.04 seconds (260x faster).

There is one behavioral change as a result of this PR. If the configuration has a syntatically invalid mbean name, instead of just failing to gather that one metric, all metrics will fail. This is because if jolokia can't parse the request, it fails the entire request. Missing mbeans/attributes still behave the way they previously have, in that an error for that specific metric will be logged, but all others will be gathered.
You can find details on this here: jolokia/jolokia#124

Closes #1328

[dylanmei]

This is huge, thanks!

[sparrc]

looks good, please rebase and remove the changelog update for now, as this will be going into 1.3

[phemmer]

looks good, please rebase and remove the changelog update for now, as this will be going into 1.3

Done

[lhoss]

looking fwd then to v1.3 release (any ETA?) !

ps:

If the configuration has a syntatically invalid mbean name, instead of just failing to gather that one metric, all metrics will fail.

I guess this is no real issue, as any such syntactic errors would be detected on the first request, and assuming the reported error/exception message contains enough information about what mbean/string contains the error.
Actually its sort of 'fail fast' (avoids getting surprises later, in case one did not test all the mbeans early).

[danielnelson]

@phemmer Just to make sure, this is still a useful update even though we are planning to merge the new jolokia2 plugin this cycle? If so, can you add back to changelog.

[phemmer]

Does "jolokia2" do bulk requests?
As long as this plugin isn't going away, I'd say the change is still useful yes. It'll be a performance boost for the users still using it.

[danielnelson]

Does "jolokia2" do bulk requests?

According to the PR it does, though I haven't looked at the code.

Let's add it to the changelog then.

[dylanmei]

Yes, jolokia2 does bulk requests also. But the plugin itself is a breaking change, I don't imagine it would get into a 1.3 version of Telegraf.

[danielnelson]

It's not breaking because it is a new plugin, right?

[phemmer]

Changelog updated.