RFR: 8036696: Add metaspace gc threshold to metaspace summary trace event

[Erik Helin]

Hi all,

this change add the metaspace GC threshold 
(Metaspace::_capacity_until_gc) to the trace event
vm/gc/heap/metaspace_summary.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8036696

Webrev:
http://cr.openjdk.java.net/~ehelin/8036696/webrev.00/

Testing:
- JFR JTREG tests
- JPRT

Thanks,
Erik