feature: add more logging to watcher

[luketchang]

Just watcher:

• Gauge: number of updates sent through UpdateHandler (number of updates checked)
• Gauge: 0 or 1 binary for double update detected
• Histogram: latency between Update event emitted and update sent through UpdateHandler (see ContractSync for example)

All agents that check home failure:

• Gauge: 0 or 1 for improper update observed (failed home; can be for all agents)
• Gauge: number of home failed checks (can be for all agents)

[yourbuddyconner]

@ltchang2019 what kind of logs do you want to add here?

[luketchang]

@yourbuddyconner updated

[luketchang]

Will turn this into broader per-agent agent_metrics field with a proper decl_metrics! macro