docs: recommend trace sampling in production (#85)

Author: lesnik512

readme.md

@@ -31,6 +31,16 @@ Authentication is intentionally omitted: pick the scheme that fits your project
 just --list
 ```
 
+## Tracing in production
+Setting `OPENTELEMETRY_ENDPOINT` turns tracing on, and by default every request is traced. In a benchmark of `GET /api/decks/{id}/`, that cut throughput by about 40%. Sampling with the standard OpenTelemetry variables brings that down to about 20%:
+
+```bash
+OTEL_TRACES_SAMPLER=parentbased_traceidratio
+OTEL_TRACES_SAMPLER_ARG=0.1
+```
+
+This keeps one trace in ten, and follows the caller's decision when a request arrives with a trace context. Errors still reach Sentry when `SENTRY_DSN` is set.
+
 ## 📝 [License](https://github.com/modern-python/fastapi-sqlalchemy-template/blob/main/LICENSE)
 
 ## Part of `modern-python`