RFR: 1478: Add metrics and loggin measuring time before WorkItems start to run [v3]

[Erik Joelsson]

> In order to investigate and assess performance issues with the Skara bots, I need more data. One key class of data I'm missing is the time it takes from when a WorkItem is created or submitted to when it actually starts to run. This patch adds both metric gauges and log messages that include this information.

Erik Joelsson has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into SKARA-1478-pending-metrics
 - Fix divider
 - Added comment
 - SKARA-1478

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Changes:
  - all: https://git.openjdk.org/skara/pull/1337/files
  - new: https://git.openjdk.org/skara/pull/1337/files/69e44eb6..4a012429

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1337&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1337&range=01-02

  Stats: 439 lines in 56 files changed: 0 ins; 0 del; 439 mod
  Patch: https://git.openjdk.org/skara/pull/1337.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1337/head:pull/1337

PR: https://git.openjdk.org/skara/pull/1337