metric: add unref_zero_count stat #9794

Job	Run time
check	2m 20s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	17m 5s
test (single-pool/cli, single-pool/program)	16m 47s
cargo-test-sbf (token/program)	9m 25s
test (token-upgrade/cli, token-upgrade/program)	14m 45s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	17m 27s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	8m 38s
cargo-test-sbf (token-upgrade/program)	7m 35s
cargo-test-sbf (feature-proposal/program)	7m 36s
cargo-test-sbf (governance/addin-mock/program, governance/program)	11m 6s
cargo-test-sbf (memo/program)	7m 5s
cargo-test-sbf (name-service/program)	6m 50s
cargo-test-sbf (stake-pool/program)	10m 21s
cargo-test-sbf (single-pool/program)	9m 9s
	2h 26m 9s

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