RFR: 8036698: Add trace event for updates to metaspace gc threshold

[Erik Helin]

Hi all,

this change adds a new trace event, vm/gc/metaspace/gc_threshold that 
tracks updates to the metaspace GC threshold 
(MetaspaceGC::_capacity_until_gc).

The event contains the old value of MetaspaceGC::_capacity_until_gc, the 
new value and also the named of the method that changed the value.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8036698

Webrev:
http://cr.openjdk.java.net/~ehelin/8036698/webrev.00/

Testing:
- JFR JTREG tests
- JPRT

Thanks,
Erik