Fix race on the LabelSet Counter (#6409) #10674

Job	Run time
build	4m 12s
lint	4m 19s
CodeQL	7m 0s
test	15m 39s
integration (requires_docker)	7m 16s
integration (integration_alertmanager)	4m 9s
integration-configs-db	3m 2s
integration (integration_backward_compatibility)	10m 20s
integration (integration_memberlist)	8m 7s
integration (integration_querier)	15m 11s
integration (integration_ruler)	5m 35s
integration (integration_query_fuzz)	4m 35s
deploy_website	49s
deploy	4m 10s
	1h 34m 24s

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