Prometheus metrics endpoint

[jpds]

Laminar should have a /metrics endpoint for Prometheus, so that dashboards can be done via Grafana.

[ohwgiles]

I don't have a need for this myself, so I won't be implementing it.

However, if someone submits a minimal PR implementing this I will consider merging it. Anyone trying this will need to add an else if branch to this function, calling back into a new function laminar.getMetrics which should be similar to getStatus