[Fleet] Datastream API is broken in serverless

[nchaulet]

Description

Our GET /api/fleet/datastreams/_stats endpoint seems broken in serverless, that API is used to populate the datastreams UI

api_not_available_exception
	Root causes:
		api_not_available_exception: Request for uri [/_data_stream/logs-*-*%2Cmetrics-*-*%2Ctraces-*-*%2Csynthetics-*-*%2Cprofiling-*/_stats?human=true] with method [GET] exists but is not available when running in serverless mode

It seems we can use the _metering/{datastreams}/stats API instead

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Added this to our next sprint.