RFR: 1478: Add metrics and loggin measuring time before WorkItems start to run [v2]

[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 incrementally with one additional commit since the last revision:

  Added comment

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Changes:
  - all: https://git.openjdk.org/skara/pull/1337/files
  - new: https://git.openjdk.org/skara/pull/1337/files/13fdfa62..69e44eb6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1337&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1337&range=00-01

  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 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