client: add Metadata.Keys method (#12805) #29625

Job	Run time
govulncheck	36s
setup-environment	27s
unittest-matrix (ubuntu-latest, ~1.24)	2m 29s
unittest-matrix (ubuntu-latest, ~1.23)	2m 23s
test-coverage	10m 42s
lint	4m 40s
checks	2m 40s
cross-build-collector (aix, ppc64)	40s
cross-build-collector (darwin, amd64)	51s
cross-build-collector (darwin, arm64)	1m 0s
cross-build-collector (linux, 386)	48s
cross-build-collector (linux, amd64)	43s
cross-build-collector (linux, arm64)	50s
cross-build-collector (linux, ppc64le)	1m 0s
cross-build-collector (linux, arm, 7)	49s
cross-build-collector (linux, s390x)	54s
cross-build-collector (windows, 386)	54s
cross-build-collector (windows, amd64)	47s
unittest	1s
	33m 14s

Provide feedback