Return 503 on hitting distributor instance limits #10565

Job	Run time
test	15m 37s
build	3m 57s
CodeQL	6m 11s
lint	4m 18s
integration-configs-db	2m 24s
integration (requires_docker)	6m 58s
integration (integration_alertmanager)	4m 9s
integration (integration_backward_compatibility)	10m 21s
integration (integration_memberlist)	7m 47s
integration (integration_querier)	15m 14s
integration (integration_ruler)	5m 51s
integration (integration_query_fuzz)	4m 34s
deploy_website	0s
deploy	0s
	1h 27m 21s

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