RFR: bot: add performance metrics for work items [v2]

[Erik Helin]

> Hi all,
> 
> please review this patch that adds runtime performance metrics for work items. I have added three new gauges:
> 
> - `skara_runner_cpu_time`
> - `skara_runner_user_time`
> - `skara_runner_allocated_bytes`
> 
> All three are per work item and make use of the `ThreadMXBean` to set the gauges. We can use the methods in `ThreadMXBean` because the `Runnable` that is running on a thread in the thread pool cannot be preempted or moved to another thread, i.e. it will have the same "carrier" thread for its whole execution.
> 
> Testing:
> - [x] Manual testing on Linux x64 using the mirror bot
> 
> Thanks,
> Erik

Erik Helin 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.

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1171/files
  - new: https://git.openjdk.java.net/skara/pull/1171/files/497d1bca..497d1bca

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1171&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1171&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1171.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1171/head:pull/1171

PR: https://git.openjdk.java.net/skara/pull/1171