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.

deploy/k8s/tavolo.yaml

@@ -71,6 +71,13 @@ data:
   # CORS_ALLOW_CREDENTIALS: "false"
   # CORS_EXPOSE_HEADERS: ""
   # CORS_MAX_AGE: "600"
+  # OpenTelemetry (kaya-otel): traces + metrics via OTLP/HTTP, disabled
+  # unless OTEL_ENABLED is set. Requires the otel extra in the image.
+  # OTEL_ENABLED: "true"
+  # OTEL_SERVICE_NAME: "tavolo"
+  # OTEL_EXPORTER_OTLP_ENDPOINT: "http://otel-collector.observability:4318"
+  # OTEL_EXPORTER_OTLP_HEADERS: "Authorization=Bearer ..."
+  # OTEL_EXCLUDED_PATHS: "/api/health"   # default; paths skipped by tracing
   # OIDC (provider lives in another namespace).
   OIDC_CLIENT_ID: tavolo
   OIDC_POST_LOGIN_REDIRECT: /