monitoring: add duration for running git commands alert

Created by: keegancsmith

It is firing a lot at the moment and quickly closes itself. We need to fully investigate why. However, for now we adjust the alert to only fire if gitserver has been underload for 5m. From looking at the graphs this problem autoresolves itself in 1m.

Part of https://github.com/sourcegraph/sourcegraph/issues/13683