Initial ZGC support

[yafernandes]

Initial support for ZGC metrics.

[jpbempel]

LGTM

[olivielpeau]

LGTM at a high level, have you been able to confirm that this config actually collects the metrics with the right types @yafernandes?

Requesting a review from @prognant as well

[prognant]

LGTM, double checked that ZGC bean names are correct.

[yafernandes]

@olivielpeau Yes, tested using OpenJDK Runtime Environment (build 14.0.1+7) on MacOS.

[vaibhavflip]

@yafernandes @jpbempel need your help to understand what is the unit of the metric collected here for pause time? milliseconds?

[jpbempel]

yes it's millisecond for CollectionTime
but regarding pause time, it depends on JDK version. for example it's pause time only for JDK14 but it includes concurrent time as well in JDK16.
for JDK17 they resolve the problem with separate MBeans: https://bugs.openjdk.java.net/browse/JDK-8265136
like in Shenandoah see #306

[vaibhavflip]

thanks @jpbempel for quick revert. we upgraded our service to jdk17, but post that, we are not getting any gc metrics for pause or collection time anymore. we are on datadog agent version 7 and using openjdk version 17.0.2-jdk-slim. please let me know if we need to do add/change something else as well

[jpbempel]

See #393

[vaibhavflip]

@jpbempel if i understand correctly, the linked refrence says zgc support is not present in jdk17, should i updat3e to jdk18 in that case?

[jpbempel]

no, it gives you a JMXFetch configuration to fetch metrics from new ZGC MBeans introduced in jdk17