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.

docker-compose.yml

@@ -122,6 +122,13 @@ services:
       CORS_ALLOW_CREDENTIALS: ${CORS_ALLOW_CREDENTIALS:-}
       CORS_EXPOSE_HEADERS: ${CORS_EXPOSE_HEADERS:-}
       CORS_MAX_AGE: ${CORS_MAX_AGE:-}
+      # OpenTelemetry (kaya-otel): disabled unless OTEL_ENABLED is set.
+      # Requires the otel extra in the image (see server/pyproject.toml).
+      OTEL_ENABLED: ${OTEL_ENABLED:-}
+      OTEL_SERVICE_NAME: ${OTEL_SERVICE_NAME:-}
+      OTEL_EXPORTER_OTLP_ENDPOINT: ${OTEL_EXPORTER_OTLP_ENDPOINT:-}
+      OTEL_EXPORTER_OTLP_HEADERS: ${OTEL_EXPORTER_OTLP_HEADERS:-}
+      OTEL_EXCLUDED_PATHS: ${OTEL_EXCLUDED_PATHS:-}
     ports:
       - "127.0.0.1:${APP_PORT:-8080}:8080"