Add process.monitor.gc.{count,elapsed} getters

[evantorrie]

Hang a gc object off process.monitor with readonly absolute values since the monitr process is started of:

1. count: # of times GC stop-the-world events have occurred
2. elapsed: cumulative elapsed time in milliseconds spent in GC

By taking a snapshot of these values at two different times (t1 and t2), application code can determine count and duration of GC events between t1 and t2.

[drewfish]

+1

[lingyan]

:cool: