util/util-jvm: Add gc pause stats for all collectors

Problem

Currently we only have gc pause stats for the ParNew collector, but
we'd like to be able to see this information for the G1 pools.

Solution

Add collection duration stats for all collector pools. Keep the jvm/gc/eden/pause_msec
stat for compatibility, but this will be the same as jvm/gc/ParNew/pause_msec.

Differential Revision: https://phabricator.twitter.biz/D1176049

doc/src/sphinx/metrics/Jvm.rst

@@ -22,12 +22,16 @@ and tenuring threshold (`jvm/tenuring_threshold`).
   A gauge of the number of the total number of collections that have occurred.
 
 **jvm/gc/eden/pause_msec**
-  A stat of the durations, in millseconds, of the eden collection pauses.
+  A stat of the durations, in millseconds, of the eden collection pauses. This is equivalent
+  to jvm/gc/ParNew/pause_msec.
 
 **jvm/gc/{gc_pool_name}/msec**
   A gauge for the named gc pool of the total elapsed time garbage collection pool
   doing collections, in milliseconds. Names are subject to change.
 
+**jvm/gc/{gc_pool_name}/pause_msec**
+  A stat for the durations, in milliseconds, of the collection pauses for the named gc pool.
+
 **jvm/gc/{gc_pool_name}/cycles**
   A gauge for the named gc pool of the total number of collections that have occurred.
   Names are subject to change.