Prometheus text-exposition backend for obs-eio

Accumulates counter/gauge/histogram deltas in-process and renders them as Prometheus text exposition format on demand, plus an optional Pushgateway client for short-lived jobs. Not a general-purpose Prometheus client and not a replacement for prometheus or prometheus-eio: it is the Prometheus backend for obs-eio's Obs_eio.backend interface, positioned for services that already use obs-eio for tracing and logging.