Doc: faq: add how to get notified with RaftMetrics #2919

Job	Run time
cluster-benchmark (cluster_benchmark)	47s
lint	6m 36s
external-stores (stores/rocksstore-v2)	6m 5s
examples (stable, raft-kv-memstore)	2m 23s
openraft-feature-test (nightly)	46s
examples (stable, raft-kv-rocksdb)	8m 23s
openraft-test (stable, 0)	3m 9s
openraft-feature-test (nightly, serde)	49s
examples (nightly, raft-kv-memstore)	2m 17s
openraft-test-bench (nightly)	53s
openraft-test (nightly, 30)	3m 14s
openraft-feature-test (nightly, single-term-leader)	50s
openraft-test (nightly, 0, single-term-leader)	2m 57s
examples (nightly, raft-kv-rocksdb)	11m 50s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	51s
stores (memstore)	42s
Build (nightly, bench,serde,bt,singlethreaded)	48s
stores (rocksstore)	5m 57s
tests-feature-test (nightly)	2m 44s
tests-feature-test (nightly, single-term-leader)	2m 43s
stores (rocksstore-compat07)	6m 1s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 42s
stores (sledstore)	54s
	1h 14m 21s

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