Prometheus metrics for workers last seen or workers heartbeat.

**Is your feature request related to a problem? Please describe.**
Right now the only way to get notified if a worker is up or down is using the boundary's api and find the worker. It's somehow frustrating since the boundary has rich metrics and telemetry and we have some alerts using alertmanager.

**Describe the solution you'd like**
That would be very nice if we had a worker heartbeat or worker last seen time metric, although I think metric's cardinality is something to consider here. That being said, it would be very helpful to have this feature.

**Describe alternatives you've considered**
To get around this issue for now, we've written a crawler that scrapes the workers' endpoint and then exposes metrics.

**Explain any additional use-cases**
-

**Additional context**
I would love to help developing this feature.

No file or test is named in the issue. Start by reviewing the workers' endpoint and Boundary's existing Prometheus metrics and telemetry, then determine how a heartbeat or last-seen metric should represent workers without excessive cardinality. Done means the metric is exposed and supports alerting on worker availability.

Written by the indexing model from the issue text.