materialize event counts in metrics endpoint

The metrics endpoint stored a lazy peewee query on the shared Prometheus collector, and collect() ran it later. Overlapping scrapes run in separate threadpool threads, so two requests could execute and step the same query's sqlite cursor at once. The rows are now read with list() before they reach the collector.

frigate/api/app.py

@@ -177,7 +177,9 @@ def metrics(request: Request):
     # Retrieve the latest statistics and update the Prometheus metrics
     stats = request.app.stats_emitter.get_latest_stats()
     # query DB for count of events by camera, label
-    event_counts: list[dict[str, Any]] = (
+    # Materialize the rows; the collector is shared across request threads and a
+    # lazy query would step one sqlite cursor from two threads.
+    event_counts: list[dict[str, Any]] = list(
         Event.select(Event.camera, Event.label, fn.Count())
         .group_by(Event.camera, Event.label)
         .dicts()