refactor: use setmetatable to set hidden variables without effecting … #17129

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	39m 13s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	36m 32s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	26m 20s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	26m 55s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	35m 33s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	36m 38s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	24m 36s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	27m 4s
	4h 12m 51s

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