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/README.md

@@ -69,6 +69,11 @@ All configuration comes from environment variables (see `.env.example`):
 | `CORS_ALLOW_CREDENTIALS` | `false` | `1`/`true`/`yes`/`on` allow cookies/credentials on cross-origin requests |
 | `CORS_EXPOSE_HEADERS` | unset | Comma-separated response headers exposed to the browser |
 | `CORS_MAX_AGE` | `600` | Seconds browsers may cache the preflight response |
+| `OTEL_ENABLED` | `false` | `1`/`true`/`yes`/`on` enable OpenTelemetry traces and metrics (requires the `otel` extra, i.e. `pip install tavolo[otel]`) |
+| `OTEL_SERVICE_NAME` | `tavolo` | `service.name` resource attribute of the exported telemetry |
+| `OTEL_EXPORTER_OTLP_ENDPOINT` | unset | Base URL of an OTLP/HTTP collector (e.g. `http://localhost:4318`); unset uses the exporter default |
+| `OTEL_EXPORTER_OTLP_HEADERS` | unset | Comma-separated `key=value` headers sent to the collector (e.g. authentication) |
+| `OTEL_EXCLUDED_PATHS` | `/api/health` | Comma-separated paths excluded from tracing and metrics (exact matches) |
 | `APP_HOST` / `APP_PORT` | `0.0.0.0` / `8080` | Bind address |
 
 ## Logging