Add recording rules for common aggregated metrics

Aaron Benton pointed out that there are cases where we often want the aggregated value of a stat - for example, n1ql_avg_req_time is really just n1ql_request_time / n1ql_requests.

If we used recording rules, we could pre-calculate them at scrape time, saving query resources.

We should look into which common aggregated stats we use and consider adding recording rules for them.