Add a metric for CPU time per thread type (#20405)

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.

scripts-dev/mypy_synapse_plugin.py

@@ -147,6 +147,7 @@ prometheus_metric_fullname_to_label_arg_map: Mapping[str, ArgLocation | None] =
     "synapse.metrics._gc.PyPyGCStats": None,
     "synapse.metrics._reactor_metrics.ReactorLastSeenMetric": None,
     "synapse.metrics.CPUMetrics": None,
+    "synapse.metrics.thread_cpu.ThreadCPUMetrics": None,
     "synapse.metrics.jemalloc.JemallocCollector": None,
     "synapse.util.metrics.DynamicCollectorRegistry": None,
     "synapse.metrics.background_process_metrics._Collector": None,