Adds a `synapse_thread_cpu_seconds_total{thread_type, mode}` metric,
grouping threads into:
1. the `main` thread
2. each registered threadpool (`threadpool_*`)
3. `tokio`
4. `other`
The kernel only reports CPU time for live threads, so the collector sums
per-thread deltas between scrapes. A thread that exits loses the CPU it
used since the last scrape.
This is primarily useful for seeing how busy the main thread is vs other
ones.
Erik Johnston
20d925bb33