RFR: bot: add performance metrics for work items

[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

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Depends on: https://git.openjdk.java.net/skara/pull/1170

Commit messages:
 - bot: add performance metrics for work items

Changes: https://git.openjdk.java.net/skara/pull/1171/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1171&range=00
  Stats: 96 lines in 2 files changed: 95 ins; 0 del; 1 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