search: measure performance of querying with nesting enabled/disabled

Created by: ijt

Feature request description

Add an endpoint that gets the top 100 recent queries from the searches table, and outputs a CSV table containing timings with and without nesting enabled.

Is your feature request related to a problem? If so, please describe.

We currently don't have a clear picture of search latency on sourcegraph.com and customer installations. It seems to be more anecdotal, based on manually running particular queries.

Describe alternatives you've considered.

One other possibility is to look at latency percentiles, but that approach doesn't show the worst case and it also doesn't tell which queries are slow.