fix: the leak of prometheus metrics #1531

Job	Run time
run ci on redhat ubi (lua-resty-worker-events, t/plugin/[a-k]*)	27m 32s
run ci on redhat ubi (lua-resty-worker-events, t/plugin/[l-z]*)	26m 22s
run ci on redhat ubi (lua-resty-worker-events, t/admin t/cli t/config-center-yaml t/control t/cor...	19m 49s
run ci on redhat ubi (lua-resty-worker-events, t/node t/pubsub t/router t/script t/secret t/strea...	19m 8s
run ci on redhat ubi (lua-resty-events, t/plugin/[a-k]*)	27m 11s
run ci on redhat ubi (lua-resty-events, t/plugin/[l-z]*)	27m 20s
run ci on redhat ubi (lua-resty-events, t/admin t/cli t/config-center-yaml t/control t/core t/deb...	19m 39s
run ci on redhat ubi (lua-resty-events, t/node t/pubsub t/router t/script t/secret t/stream-node ...	18m 23s
	3h 5m 24s

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