x/build: set up Gerrit latency monitoring

[bradfitz]

I regularly encounter Gerrit latency & server errors lately but my bug reports to the team are rarely found to be detailed enough to be actionable or reproducible, or are viewed as anecdotal.

We should probably start gathering our own data on Gerrit latency & reliability with enough data to file actionable bug reports.

We could also use said data to see if Gerrit is getting better or worse over time, and to inform any migration decisions we make in the future.

Maybe just set up our own Stackdriver probers from a number of regions?

/cc @dmitshur