Throttler: `SelfMetric` interface, simplify adding new throttler metrics #356

Job	Run time
Set push variable	1s
Build and push vttestserver (mysql80)	1m 52s
Build and push lite (latest)	2m 14s
Build and push lite (percona80)	2m 8s
Build and push (bullseye, vtadmin)	1m 17s
Build and push (bullseye, vtorc)	41s
Build and push (bullseye, vtgate)	40s
Build and push (bullseye, vttablet)	39s
Build and push (bullseye, mysqlctld)	43s
Build and push (bullseye, mysqlctl)	43s
Build and push (bullseye, vtctl)	40s
Build and push (bullseye, vtctlclient)	39s
Build and push (bullseye, vtctld)	1m 18s
Build and push (bullseye, vtctldclient)	37s
Build and push (bullseye, logrotate)	27s
Build and push (bullseye, logtail)	31s
Build and push (bullseye, vtbackup)	39s
Build and push (bullseye, vtexplain)	42s
Build and push (bookworm, vtadmin)	1m 12s
Build and push (bookworm, vtorc)	42s
Build and push (bookworm, vtgate)	46s
Build and push (bookworm, vttablet)	36s
Build and push (bookworm, mysqlctld)	27s
Build and push (bookworm, mysqlctl)	28s
Build and push (bookworm, vtctl)	40s
Build and push (bookworm, vtctlclient)	39s
Build and push (bookworm, vtctld)	1m 16s
Build and push (bookworm, vtctldclient)	39s
Build and push (bookworm, logrotate)	27s
Build and push (bookworm, logtail)	16s
Build and push (bookworm, vtbackup)	39s
Build and push (bookworm, vtexplain)	40s
	25m 58s

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