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/.env.example

@@ -61,6 +61,22 @@ TURN_TIMEOUT_SECONDS=30
 # Seconds browsers may cache the preflight response (default 600):
 #CORS_MAX_AGE=600
 
+# OpenTelemetry (via kaya-otel + the contrib system-metrics package). Off
+# unless OTEL_ENABLED is truthy; exports traces, HTTP/app/process metrics
+# and logs over OTLP/HTTP (requires the tavolo-app "otel" extra, bundled
+# in the Docker image). The endpoint defaults to the standard
+# OTEL_EXPORTER_OTLP_ENDPOINT (localhost:4318) when unset.
+#OTEL_ENABLED=true
+#OTEL_SERVICE_NAME=tavolo
+#OTEL_EXPORTER_OTLP_ENDPOINT=http://otel-collector:4318
+#OTEL_EXPORTER_OTLP_HEADERS=Authorization=Bearer token,X-Tenant=one
+# Paths excluded from tracing and metrics (default /api/health).
+#OTEL_EXCLUDED_PATHS=/api/health
+# Metric export interval (ms, default 60000) and session/game count poll
+# interval (seconds, default 15).
+#OTEL_METRIC_EXPORT_INTERVAL_MILLIS=60000
+#OTEL_METRICS_POLL_INTERVAL_SECONDS=15
+
 # App server
 APP_HOST=0.0.0.0
 APP_PORT=8080