Add information to slow FTS queries for debugging

Current slow operations for FTS queries are logged like the below:

[WARN] slow-query: index: INDEX_NAME, query: <ud>{"ctl":{"timeout":48190},"fields":["..."],"from":6100,"query":{...},"size":100,"sort":[...]}</ud>, resultset bytes: 0, duration: 5.144471563s, err: query request rejected -- rest.(*QueryHandler).ServeHTTP() at rest_index.go:306 

Would be good to have the source username and address for the queries to identify where they are coming from. Also, we can add further information like the user agent if available or even something similar to what is done in completed_requests for the Query Service.