Return 503 on hitting distributor instance limits #10563

Job	Run time
lint	4m 13s
build	4m 2s
test	15m 35s
CodeQL	6m 27s
integration-configs-db	2m 27s
integration (requires_docker)	7m 6s
integration (integration_alertmanager)	3m 58s
integration (integration_backward_compatibility)	10m 44s
integration (integration_memberlist)	7m 36s
integration (integration_querier)	15m 22s
integration (integration_ruler)	5m 49s
integration (integration_query_fuzz)	5m 3s
deploy_website	0s
deploy	0s
	1h 28m 22s

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