Currently, the gc+heap=info log lines for GenCollectedHeap look as follows:

Serial:

[10.838s][info][gc,heap ] GC(29) DefNew: 551835K->1306K(619328K)
[10.838s][info][gc,heap ] GC(29) Tenured: 1166K->1166K(1376256K)

CMS:

[30.189s][info][gc,heap ] GC(326) ParNew: 552345K->1814K(619328K)
[30.189s][info][gc,heap ] GC(326) CMS: 1325K->1325K(1376256K)

Similarly to JDK-8223575, it will be vey helpful to add size transitions for the young gen sub spaces (at least Eden and From-Space).

While I'm at it, I'll also add the capacity before the GC (again, similar to the change for JDK-8223575) as, in certain situations, it can change during the GC.