RFR: After GC reporting should also report metaspace progress

[Zhengyu Gu]

Following upstream discussion, Shenandoah should follow other GCs to 
report metaspace progress after GCs.

I also changed logging tag for heap from (gc, ergo) to (gc, heap) to be 
consistent with others.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/metaspace_stats/webrev.00/

Test:

   tier2_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu