fix: the leak of prometheus metrics #15739

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	23m 28s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	23m 59s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	17m 14s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	18m 2s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	24m 38s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	25m 21s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	18m 11s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	18m 7s
	2h 49m 0s

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