RFR: Add metrics for RestRequestCache

[Erik Helin]

On Tue, 25 May 2021 17:29:10 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> 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.

Looks good!

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Marked as reviewed by ehelin (Reviewer).

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