Replaces #6362.
I've reduced the attributes to only record the gc name and the action
that was taken (i.e. I've removed the gc cause). If needed we can add
the cause later, but for now this should be sufficient to determine
total time spent in GC, and categorize time spent as stop the world or
parallel.
In the 10/27 java sig we discussed that it would be valuable to
enumerate the attributes reported for memory pool and gc metrics when
different gcs are used.
I've went ahead and added a readme for the runtime metrics which
includes detailed information on the attributes reported. Note that I
also have the same data for gc metrics added in #6964 and #6963, but
will wait to add until those PRs are merged.
Eric Sirianni
Aaron Ai
jack-berg