Add optional OpenTelemetry integration via kaya-otel

Adds an OTEL_ENABLED opt-in that assembles kaya-otel's OTelMixin into
the app, exporting HTTP/WebSocket traces and request metrics to an
OTLP/HTTP collector. Configured through OTEL_SERVICE_NAME,
OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_HEADERS and
OTEL_EXCLUDED_PATHS (defaults to the /api/health probe endpoint).

kaya-otel is an optional 'otel' extra imported lazily, so default
installs and the test suite do not need the OpenTelemetry packages.
The kaya dependencies are bumped to 0.0.4, which kaya-otel requires.

server/tests/test_config.py

@@ -130,5 +130,40 @@ class CorsSettingsTests(unittest.TestCase):
         self.assertEqual(3600, settings.cors_max_age)
 
 
+class OTelSettingsTests(unittest.TestCase):
+    def test_otel_disabled_by_default(self):
+        settings = _settings({})
+        self.assertFalse(settings.otel_enabled)
+        self.assertEqual("tavolo", settings.otel_service_name)
+        self.assertIsNone(settings.otel_exporter_endpoint)
+        self.assertIsNone(settings.otel_exporter_headers)
+
+    def test_otel_enabled_parses_boolean(self):
+        for value in ("1", "true", "TRUE", "yes", "on"):
+            self.assertTrue(_settings({"OTEL_ENABLED": value}).otel_enabled)
+        for value in ("0", "false", "no", "off", "anything-else"):
+            self.assertFalse(_settings({"OTEL_ENABLED": value}).otel_enabled)
+
+    def test_otel_settings_are_passed_through(self):
+        settings = _settings({
+            "OTEL_SERVICE_NAME": "cards",
+            "OTEL_EXPORTER_OTLP_ENDPOINT": "http://collector:4318",
+            "OTEL_EXPORTER_OTLP_HEADERS": "Authorization=Bearer t, X-Tenant=one",
+        })
+        self.assertEqual("cards", settings.otel_service_name)
+        self.assertEqual("http://collector:4318", settings.otel_exporter_endpoint)
+        self.assertEqual(
+            ("Authorization=Bearer t", "X-Tenant=one"),
+            settings.otel_exporter_headers,
+        )
+
+    def test_otel_excluded_paths_defaults_to_health_endpoint(self):
+        self.assertEqual(("/api/health",), _settings({}).otel_excluded_paths)
+
+    def test_otel_excluded_paths_parses_comma_separated_list(self):
+        settings = _settings({"OTEL_EXCLUDED_PATHS": "/api/health, /metrics"})
+        self.assertEqual(("/api/health", "/metrics"), settings.otel_excluded_paths)
+
+
 if __name__ == "__main__":
     unittest.main()