search: add monitoring & dashboard for requests by status (success, error, timeout, partial timeout)

Created by: slimsag

Before: Site admins had no way to know if the majority of their users searches were ending in errors, timeouts, or were actually successful.

After: Site admins can visit the searcher Grafana dashboard and easily see how many searches are ending in success/error/timeout/partial timeout:

In particular, this would've helped us to determine the impact of #6754 (closed) for two customers and identify whether it was a minor problem or a larger and separate issue.

Additionally, this fixes an oversight I made in https://github.com/sourcegraph/sourcegraph/pull/7020#discussion_r354085645

Fixes #6756 (closed)

Test plan: Manually tested