Add simple prober to monitoring

[yolocs]

To continuously monitor JVS liveness, we need a basic prober. The basic logic is:

0. Give a JVS API endpoint
1. Request a token
2. Validate the token
3. Write a success metric data point
4. Repeat after a while

We then set up alerting based on the success metric.

We already have jvsctl that's capable of requesting/validating token. We can build an image on top of it. We can use log-based metrics. We can use cloud scheduler for the repeat.

[yolocs]

Can we close this now? @sailorlqh

[sailorlqh]

Can we close this now? @sailorlqh

I will close this once #243 is merged. @yolocs

[sailorlqh]

Fixed by #243