movistar_u7d.py, movistar_epg.py & grafana-dashboard.json:

 - Since prometheus metrics are severely limited when working in
 multiprocess mode, move the registry to our state backend, and push
 events to it, proxying back export metrics. This way we get a live
 view of streaming clients, ordered visually by their initial latency
 and where we can see the channel, the client ip, the exact url for
 catchup streams, as well as the start and end times.

 - Far from impacting the measured latencies, thanks to pushing
 events with asyncio.create_task() and moving logging to the epg
 backend, I've seen catchup streams opened in as low as 281ms.

Signed-off-by: Javier Marcet <javier@marcet.info>