Export app, process and log telemetry when OTEL_ENABLED is set

Adds tavolo.active_users and tavolo.ongoing_games gauges, process/CPython
metrics via opentelemetry-instrumentation-system-metrics and OTLP log
export. The game store tracks ongoing matches (Redis sorted set, in-memory
set) and the session stores expose live counts for the gauges; a poller
refreshes them since metric callbacks cannot await the stores.

server/tests/test_otel.py

@@ -69,6 +69,15 @@ class OTelMixinFromSettingsTests(unittest.TestCase):
         self.assertIsNone(mixin.kwargs["headers"])
         self.assertEqual(("/api/health",), mixin.kwargs["excluded_paths"])
 
+    def test_meter_provider_is_passed_through(self):
+        with _stub_kaya_otel():
+            provider = object()
+            mixin = otel_mixin_from_settings(
+                _settings({"OTEL_ENABLED": "1"}), meter_provider=provider
+            )
+        assert isinstance(mixin, _StubOTelMixin)
+        self.assertIs(provider, mixin.kwargs["meter_provider"])
+
     def test_excluded_paths_are_passed_through(self):
         with _stub_kaya_otel():
             mixin = otel_mixin_from_settings(_settings({