Fix labelset race condition #11198

Job	Run time
test-no-race	7m 34s
test-no-race	7m 34s
lint	4m 25s
lint	4m 25s
test	36m 58s
test	16m 13s
build	4m 13s
build	4m 13s
CodeQL	7m 0s
CodeQL	7m 0s
integration-configs-db	2m 46s
integration-configs-db	2m 46s
integration (requires_docker)	7m 7s
integration (requires_docker)	7m 7s
integration (integration_alertmanager)	4m 9s
integration (integration_alertmanager)	4m 9s
integration (integration_backward_compatibility)	10m 21s
integration (integration_backward_compatibility)	10m 21s
integration (integration_memberlist)	7m 23s
integration (integration_memberlist)	7m 23s
integration (integration_querier)	15m 18s
integration (integration_querier)	15m 18s
integration (integration_ruler)	5m 52s
integration (integration_ruler)	5m 52s
integration (integration_query_fuzz)	5m 17s
integration (integration_query_fuzz)	5m 17s
deploy_website	0s
deploy_website	0s
deploy	0s
deploy	0s
	3h 36m 1s

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