stats key returns a stat as non-key value in violation of the protocol

The protocol calls for all stats to be returned as a key and value pair. Accordingly, client libraries parse for key and value pairs.

According to the new docs for stats key, one important stat returns only a value. This would make most client libraries fail to parse the response.

See: https://github.com/couchbase/ep-engine/blob/master/docs/stats.org#29-stats-key-and-vkey