docs(otel): document sampling configuration and tail-sampling guidance

pkg/gotenberg/internal/otel/doc.go

@@ -3,5 +3,22 @@
 //
 // Significantly inspired by https://github.com/lucavallin/gotel.
 //
+// # Sampling
+//
+// No sampler is configured in code, so the SDK default applies:
+// parentbased_always_on. Every trace is recorded and exported. Operators tune
+// sampling through the standard environment variables, honored by the SDK:
+//
+//	OTEL_TRACES_SAMPLER       e.g. parentbased_traceidratio, always_off
+//	OTEL_TRACES_SAMPLER_ARG   e.g. 0.1 for a 10% ratio
+//
+// Head sampling drops whole traces up front, including the rare slow or failed
+// conversions that matter most for diagnosis. For high-throughput deployments,
+// prefer keeping head sampling permissive and applying tail sampling in the
+// collector (sample on error or high latency), which decides after a trace
+// completes. Gotenberg emits trace-based metric exemplars, so the conversion
+// histograms still link to representative traces regardless of the head
+// sampling ratio.
+//
 // See https://opentelemetry.io/.
 package otel