Add users stats http api to ingester #228

Job	Run time
build	3m 54s
test	15m 16s
lint	2m 40s
CodeQL	6m 47s
integration-configs-db	2m 14s
integration (requires_docker)	6m 19s
integration (integration_alertmanager)	4m 3s
integration (integration_backward_compatibility)	8m 54s
integration (integration_memberlist)	8m 7s
integration (integration_querier)	12m 13s
integration (integration_ruler)	4m 37s
integration (integration_query_fuzz)	2m 32s
deploy_website	0s
deploy	0s
	1h 17m 36s

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