Add users stats http api to ingester #226

Job	Run time
CodeQL	6m 33s
build	3m 53s
lint	4m 5s
test	14m 56s
integration-configs-db	2m 15s
integration (requires_docker)	6m 16s
integration (integration_alertmanager)	3m 55s
integration (integration_backward_compatibility)	9m 33s
integration (integration_memberlist)	7m 29s
integration (integration_querier)	12m 26s
integration (integration_ruler)	4m 30s
integration (integration_query_fuzz)	2m 32s
deploy_website	0s
deploy	0s
	1h 18m 23s

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