Re-enable useful alerts

[oliverchang]

We need to reenable and fix up our alerts.

Some ideas:

• Dynamic alert for bot count based on clusters.yaml.
• Extend fuzzing time metric to engine based fuzzers as well.

We should also evaluate and delete any metrics that we no longer use.

[oliverchang]

Looks like there is finally an alerts API available that lets us manage them progamatically: https://cloud.google.com/monitoring/alerts/using-alerting-api

One thing we can do here is to create alerts for bot count where the threshold is dynamically based on the expected number of bots (based on clusters.yaml).