G1 Support

Implement support for all G1 stop the world pauses and sub-events. A total of 61 sub_metrics related to G1 GC are supported:

* g1-pause-young
* g1-pause-mixed
* g1-pause-remark
* g1-pause-cleanup
* g1-pause-remark.ref-proc
* g1-pause-young.parallel
* g1-pause-young.parallel.gcworkers
* g1-pause-young.parallel.ext-root-scanning.avg
* g1-pause-young.parallel.ext-root-scanning.max
* g1-pause-young.parallel.update-rs.avg
* g1-pause-young.parallel.update-rs.max
* g1-pause-young.parallel.update-rs.processed-buffers.avg
* g1-pause-young.parallel.update-rs.processed-buffers.max
* g1-pause-young.parallel.scan-rs.avg
* g1-pause-young.parallel.scan-rs.max
* g1-pause-young.parallel.object-copy-rs.avg
* g1-pause-young.parallel.object-copy-rs.max
* g1-pause-young.parallel.termination.avg
* g1-pause-young.parallel.termination.max
* g1-pause-young.parallel.gc-worker-other.avg
* g1-pause-young.parallel.gc-worker-other.max
* g1-pause-young.parallel.gc-worker-total.avg
* g1-pause-young.parallel.gc-worker-total.max
* g1-pause-young.parallel.gc-worker-end.avg
* g1-pause-young.parallel.gc-worker-end.max
* g1-pause-young.code-root-fixup
* g1-pause-young.clear-ct
* g1-pause-young.other
* g1-pause-young.other.choose-cset
* g1-pause-young.other.ref-proc
* g1-pause-young.other.reg-enq
* g1-pause-young.other.free-cset
* g1-pause-mixed.parallel
* g1-pause-mixed.parallel.gcworkers
* g1-pause-mixed.parallel.ext-root-scanning.avg
* g1-pause-mixed.parallel.ext-root-scanning.max
* g1-pause-mixed.parallel.update-rs.avg
* g1-pause-mixed.parallel.update-rs.max
* g1-pause-mixed.parallel.update-rs.processed-buffers.avg
* g1-pause-mixed.parallel.update-rs.processed-buffers.max
* g1-pause-mixed.parallel.scan-rs.avg
* g1-pause-mixed.parallel.scan-rs.max
* g1-pause-mixed.parallel.object-copy-rs.avg
* g1-pause-mixed.parallel.object-copy-rs.max
* g1-pause-mixed.parallel.termination.avg
* g1-pause-mixed.parallel.termination.max
* g1-pause-mixed.parallel.gc-worker-other.avg
* g1-pause-mixed.parallel.gc-worker-other.max
* g1-pause-mixed.parallel.gc-worker-total.avg
* g1-pause-mixed.parallel.gc-worker-total.max
* g1-pause-mixed.parallel.gc-worker-end.avg
* g1-pause-mixed.parallel.gc-worker-end.max
* g1-pause-mixed.code-root-fixup
* g1-pause-mixed.clear-ct
* g1-pause-mixed.other
* g1-pause-mixed.other.choose-cset
* g1-pause-mixed.other.ref-proc
* g1-pause-mixed.other.reg-enq
* g1-pause-mixed.other.free-cset
* g1-pause-young.parallel.gc-worker-start.avg
* g1-pause-young.parallel.gc-worker-start.max
* g1-pause-mixed.parallel.gc-worker-start.avg
* g1-pause-mixed.parallel.gc-worker-start.max