Prometheus /metrics handler.

Returns a handler that renders the instrument registry in Prometheus text exposition format. Livery's livery_instrument_metrics middleware already records HTTP server metrics into that registry; mount this on a route to expose them:

R1 = livery_router:add('GET', <<"/metrics">>, livery_metrics:handler(), #{}, R0).

The body and Content-Type (text/plain; version=0.0.4; charset=utf-8) come from instrument_prometheus. Requires the instrument application to be running (it is, as a Livery dependency).