movistar_u7d.py: since we are taking measure of latencies, expose

them as Prometheus /metrics, segregated by channel and whether live
or catchup streams. This will be great to analyze stream response
times, for example with:

 sanic_request_latency_sec_sum{method='catchup'}/sanic_request_latency_sec_count{method='catchup'}

&

 sanic_request_latency_sec_sum{method='live'}/sanic_request_latency_sec_count{method='live'}

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