Use ParentBased sampler to respect parent span sampling decision

docs/content/reference/install-configuration/observability/tracing.md

@@ -41,7 +41,7 @@ tracing: {}
 | <a id="opt-tracing-addInternals" href="#opt-tracing-addInternals" title="#opt-tracing-addInternals">`tracing.addInternals`</a> | Enables tracing for internal resources (e.g.: `ping@internal`).                                                                                                             | false                               | No       |
 | <a id="opt-tracing-serviceName" href="#opt-tracing-serviceName" title="#opt-tracing-serviceName">`tracing.serviceName`</a> | Defines the service name resource attribute.                                                                                                                                | "traefik"                           | No       |
 | <a id="opt-tracing-resourceAttributes" href="#opt-tracing-resourceAttributes" title="#opt-tracing-resourceAttributes">`tracing.resourceAttributes`</a> | Defines additional resource attributes to be sent to the collector. See [resourceAttributes](#resourceattributes) for details.                                              | []                                  | No       |
-| <a id="opt-tracing-sampleRate" href="#opt-tracing-sampleRate" title="#opt-tracing-sampleRate">`tracing.sampleRate`</a> | The proportion of requests to trace, specified between 0.0 and 1.0.                                                                                                         | 1.0                                 | No       |
+| <a id="opt-tracing-sampleRate" href="#opt-tracing-sampleRate" title="#opt-tracing-sampleRate">`tracing.sampleRate`</a> | The proportion of requests to trace, specified between 0.0 and 1.0.<br /> Since Traefik supports parent-based sampling ratios, root spans (i.e., spans initiated by Traefik) are sampled according to this rate, while child spans inherit the sampling decision of their parent (i.e., the tracing context from incoming requests). See [sampleRate](#samplerate) for details.  | 1.0                                 | No       |
 | <a id="opt-tracing-capturedRequestHeaders" href="#opt-tracing-capturedRequestHeaders" title="#opt-tracing-capturedRequestHeaders">`tracing.capturedRequestHeaders`</a> | Defines the list of request headers to add as attributes.<br />It applies to client and server kind spans.                                                                  | []                                  | No       |
 | <a id="opt-tracing-capturedResponseHeaders" href="#opt-tracing-capturedResponseHeaders" title="#opt-tracing-capturedResponseHeaders">`tracing.capturedResponseHeaders`</a> | Defines the list of response headers to add as attributes.<br />It applies to client and server kind spans.                                                                 | []                                  | False    |
 | <a id="opt-tracing-safeQueryParams" href="#opt-tracing-safeQueryParams" title="#opt-tracing-safeQueryParams">`tracing.safeQueryParams`</a> | By default, all query parameters are redacted.<br />Defines the list of query parameters to not redact.                                                                     | []                                  | No       |
@@ -61,6 +61,17 @@ tracing: {}
 | <a id="opt-tracing-otlp-grpc-tls-key" href="#opt-tracing-otlp-grpc-tls-key" title="#opt-tracing-otlp-grpc-tls-key">`tracing.otlp.grpc.tls.key`</a> | This instructs the exporter to send the tracing to the OpenTelemetry Collector using HTTP.<br /> Setting the sub-options with their default values.                         | ""null/false ""                     | No       |
 | <a id="opt-tracing-otlp-grpc-tls-insecureskipverify" href="#opt-tracing-otlp-grpc-tls-insecureskipverify" title="#opt-tracing-otlp-grpc-tls-insecureskipverify">`tracing.otlp.grpc.tls.insecureskipverify`</a> | If `insecureSkipVerify` is `true`, the TLS connection to the OpenTelemetry Collector accepts any certificate presented by the server regardless of the hostnames it covers. | false                               | Yes      |
 
+## sampleRate
+
+The `sampleRate` option controls trace sampling using a `ParentBased(TraceIDRatioBased)` strategy.
+
+!!! info "Sampling Strategy Behavior"
+
+    - **Root spans** (trace originating at Traefik): Sampled according to the configured `sampleRate` using trace ID ratio-based sampling.
+    - **Child spans** (requests with existing trace context): Inherit the sampling decision from the parent span, regardless of the local `sampleRate`.
+
+    This ensures consistent sampling decisions across distributed traces: once a trace is sampled, all spans in that trace are sampled, providing complete end-to-end visibility.
+
 ## resourceAttributes
 
 The `resourceAttributes` option allows setting the resource attributes sent along the traces.

pkg/observability/types/tracing.go

@@ -97,7 +97,7 @@ func (c *OTelTracing) Setup(ctx context.Context, serviceName string, sampleRate
 	// span processor to aggregate spans before export.
 	bsp := sdktrace.NewBatchSpanProcessor(exporter)
 	tracerProvider := sdktrace.NewTracerProvider(
-		sdktrace.WithSampler(sdktrace.TraceIDRatioBased(sampleRate)),
+		sdktrace.WithSampler(sdktrace.ParentBased(sdktrace.TraceIDRatioBased(sampleRate))),
 		sdktrace.WithResource(res),
 		sdktrace.WithSpanProcessor(bsp),
 	)