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.

deploy/k8s/tavolo.yaml

@@ -71,13 +71,16 @@ 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. The image bundles the otel extra.
+  # OpenTelemetry (kaya-otel + system-metrics): traces, metrics and logs
+  # via OTLP/HTTP, disabled unless OTEL_ENABLED is set. The image bundles
+  # the otel extra.
   # 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
+  # OTEL_METRIC_EXPORT_INTERVAL_MILLIS: "60000"
+  # OTEL_METRICS_POLL_INTERVAL_SECONDS: "15"
   # OIDC (provider lives in another namespace).
   OIDC_CLIENT_ID: tavolo
   OIDC_POST_LOGIN_REDIRECT: /