Track JVM gc time

[jack-berg]

Related to #2900.

Its valuable to track the time spent in garbage collection, how many garbage collection events took place, and the type of garbage collection events that took place (different garbage collector implementations have various phases which happen concurrently or in parallel).

I proposed here that we use a histogram to collect this data:

Name	Description	Unit	Unit UCUM	Instrument Type	Value Type	Attribute Key	Attribute Values
process.runtime.jvm.gc.time	Time spent performing JVM garbage collection actions	Milliseconds	ms	Histogram	Int64	gc	Name of garbage collector
						action	The collection action performed

Recording gc event times as a histogram would enable the following analysis:

• Total time spent on garbage collection.
• Count of garbage collection events.
• Min, max, average, percentiles of garbage collection time events.
• The attributes allow you to characterize gc time by the types of events. With a little prior knowledge you can analyze all gc spent concurrently vs stop the world.

[carlosalberto]

Similarly to #2900, it makes sense given the existing Java GC metrics - I'm mostly curious about the "action" attribute as it's not well explained in the Java docs themselves, so your examples help (and even more examples would be appreciated).

[carlosalberto]

cc @jsuereth @jmacd