[metrics] Add total-gc-time and adjust other metrics
This change introduces the following:
1) Added total-gc-collection-time in ms as a counter so that, at anytime,
we can know how much time is spent in GC.
2) Added MetricsAverage to report average of the given data point.
3) Changed mutator-paue-time to average, which makes more sense. Also it
is renamed to world-stop-time.
4) Added averages of gc-throughputs.
5) Removed gc-meta-data-size metric as we don't capture it normally due
to the high cost of capturing this data.
Test: Observe pitot data
Bug: 191404436
Change-Id: I9da7f8c588ac4b42414beedb1b4004e0ac4b5fc2
6 files changed