fix: use max_req_body_bytes field in custom_format #17124

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	35m 14s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	34m 36s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	25m 16s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	26m 57s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	34m 50s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	36m 36s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	26m 13s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	27m 33s
	4h 7m 15s

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