put new gc metrics behind a flag

[arbll]

In 6.6 we introduced new names for gc metrics :

• jvm.gc.minor_collection_count
• jvm.gc.minor_collection_time
• jvm.gc.major_collection_count
• jvm.gc.major_collection_time

This names make more sense but unfortunately due to some gotchas in the way jmxfetch collect metrics this also had the effect of not sending the metrics using the old names.

This PR fixes this by putting the new names behind a flag.

[olivielpeau]

(tracking this flag as something we could want to enable by default in version 7 of the Agent)

[truthbk]

CircleCI failure is unrelated and fixed in alternative PR.

This looks good! Thank you! 🏗

[olivielpeau]

Had a couple of questions/notes:

• how would this PR affect JMXFetch in the APM java agent?
• (not urgent): we should document this new option in the jmx.d/conf.yaml.example file that's shipped with the agent, and also in all the example conf files of integrations

[truthbk]

Synced with APM, they think it's fine, but they will schedule some work on their side to make sure they have full compatibility with either collection mode.