Label leaking of thread metrics

[5kbpers]

Bug Report

Here we buffered all labels (thread IDs), then labels can not be dropped even if threads have been exited.

tikv/components/tikv_util/src/metrics/threads_linux.rs

Lines 25 to 40 in 13db52e

	struct Metrics {
	cpu_totals: CounterVec,
	io_totals: CounterVec,
	threads_state: IntGaugeVec,
	voluntary_ctxt_switches: IntCounterVec,
	nonvoluntary_ctxt_switches: IntCounterVec,
	}
	/// A collector to collect threads metrics, including CPU usage
	/// and threads state.
	struct ThreadsCollector {
	pid: pid_t,
	descs: Vec<Desc>,
	metrics: Mutex<Metrics>,
	tid_retriever: Mutex<TidRetriever>,
	}