Rate events over at least one second (#24455)

* Rate events over at least one second

EventsPerSecond.eps() divided the event count by the time since start(),
which can be a few milliseconds right after a restart. Frames buffered
during an ffmpeg restart then report as 100+ fps, and the same happens to
the detector fps. Use a window of at least one second.

* Keep sub-second windows consistent

Floor the divisor at the window length when the window is shorter than a
second, so a caller with a sub-second window still gets its true rate.

frigate/util/builtin.py

@@ -56,10 +56,13 @@ class EventsPerSecond:
             self._start = now
         # compute the (approximate) events in the last n seconds
         self.expire_timestamps(now)
-        seconds = min(now - self._start, self._last_n_seconds)
-        # avoid divide by zero
-        if seconds == 0:
-            seconds = 1
+        # rate over at least one second (or the whole window, if shorter),
+        # so a burst of events right after start() is not divided by a
+        # tiny window
+        seconds = max(
+            min(now - self._start, self._last_n_seconds),
+            min(1.0, self._last_n_seconds),
+        )
         return len(self._timestamps) / seconds
 
     # remove aged out timestamps