Incorrect naming for the `elasticsearch_jvm_gc_collection_seconds_count` metric

[anguswilliams]

The metric elasticsearch_jvm_gc_collection_seconds_count is incorrectly named, it counts GC events rather than GC time, so should be named elasticsearch_jvm_gc_collection_count.

This issue was fixed by this MR:
#324

But then reverted in this MR, due to it being a breaking change:
#430

Could this naming fix be re-added into an appropriate release cycle?