x/build: collect key metrics from the build infrastructure

[cagedmantis]

This is a tracking issue for the collection of key operational metrics from the build infrastructure. These metrics are being collected for the following reasons:

• Increasing speed of root cause analysis when an issue arrises.
• Understanding how changes correlate to performance changes.
• Facilitating key areas for possible optimizations.
• Facilitating monitoring and alerting on metrics.

The task list below will be appended to once a detailed list of key metrics are identified.

• Collect Metrics

• Create Dashboards

• GCP Aggregate Service API metrics

• AWS Aggregate Service API

• GitHub Aggregate Service API

• Gerrit Aggregate Service API

• TLS certificate lifetime

• General OS/Application/Container specific metrics

Coordinator

• Amount of time waiting for VM quota
• Buildlet creation latency by stage and type
• Total buildlet creation latency
• VM instance creation failures
• Instance creation queue depth
• Instance creation queue latency
• Active Trybot count, latency, failures by type
• Buildlet count by pool
• Pending build count by type
• Pending build latency by type
• Uptime
• Build rate
• General API instrumentation (like ochttp)

Gomote

• Sessions created
• Sessions destroyed
• Session duration
• Command usage (SSH, put, etc.)

@golang/release