Fix various runtime metrics issues

[vlovgr]

Three separate issues discovered while setting up dashboards.

• d651367 The jvm.gc.duration was reported in milliseconds instead of seconds because:

  MillisPerSecond = TimeUnit.MILLISECONDS.toNanos(1) = 1000000L
  

  instead of the expected milliseconds per seconds.

  MillisPerSecond = TimeUnit.SECONDS.toMillis(1) = 1000L
  

• 5dffacc The jvm.class.count (classes currently loaded) and jvm.class.loaded (classes loaded since start) were accidentally swapped.
• 1e7a587 For scalanative.gc.duration the accumulated collection time was recorded with every refresh (and reported in milliseconds, similar to the first issue). We now record every collection since the previous refresh, counting the average duration (in seconds) of the collections in the refresh period.

[iRevive]

Thanks!