agent: expose metrics

[lucab]

update-agent is typically running on all CL nodes, watching and reacting to update-engine events and annotation. It should expose its status as metrics that can be analyzed and alerted upon. Access to those should be governed by kubernetes RBAC policies.

This is a preliminary list of interesting metrics:

• go runtime stats
• update-engine status
• update-engine last checked time
• reboot-needed state
• reboot-in-progress state
• CL version and channel