feat: expose pending messages metric in async pipeline (#2330) #1265

Job	Run time
build-rust-amd64	6m 15s
build-rust-arm64	6m 46s
Build binaries	7m 0s
Build & push linux/amd64 and linux/arm64 (numaflow)	2m 13s
Bom	1m 11s
Release	19s
	23m 44s

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