Add Prometheus exporter for go2rtc and Frigate stream health

Standalone Python exporter that polls go2rtc /api/streams and Frigate
/api/stats every 15s, tracking bytes_recv deltas to detect stalled
streams. Exposes metrics on port 9199 including stream_up,
bytes_per_second, consumers, and camera_fps.

monitoring/requirements.txt

@@ -0,0 +1,3 @@
+prometheus-client>=0.20.0
+requests>=2.31.0
+pyyaml>=6.0