(PDB-353) Don't add new mbeans for metrics URLs.

- Added a check to ensure that requests for metrics/mbeans information
  do not cause creation of additional mbeans to track the metrics URLs.
  We check specifically for the "available mbeans list" URL, and for any
  specific metric URL from that list.
- Added test code to verify that :timers and :meters are not created
  when metrics-related URLs are accessed.
I don't think this will hold up if a url-prefix is used.  The endpoint could be at /foo/v4/metrics/mbeans for instance.