Missing monitoring for when gitserver fetches/execs/clones are slow

Created by: slimsag

monitoring these two failure cases would be useful (I've seen them occur on customer deployments and indicate real problems): https://sourcegraph.com/github.com/sourcegraph/sourcegraph@45ee47a084694eecbd6b6a4c56e150645bc5a092/-/blob/cmd/gitserver/server/server.go#L596-601