RFR: Add metrics for RestRequestCache

[Erik Joelsson]

This change adds some metrics to better understand what the RestRequestCache is doing and to be able to monitor the consequences when I make changes to it.

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Commit messages:
 - Add metrics for RestRequestCache

Changes: https://git.openjdk.java.net/skara/pull/1166/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1166&range=00
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1166.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1166/head:pull/1166

PR: https://git.openjdk.java.net/skara/pull/1166