feat(core): Add inheritOrSampleWith helper to traceSampler

dev-packages/node-integration-tests/suites/tracing/sample-rate-propagation/tracesSampler/server.js

@@ -4,12 +4,8 @@ const Sentry = require('@sentry/node');
 Sentry.init({
   dsn: 'https://[email protected]/1337',
   transport: loggingTransport,
-  tracesSampler: ({ parentSampleRate }) => {
-    if (parentSampleRate) {
-      return parentSampleRate;
-    }
-
-    return 0.69;
+  tracesSampler: ({ inheritOrSampleWith }) => {
+    return inheritOrSampleWith(0.69);
   },
 });
 

packages/core/src/tracing/sampling.ts

@@ -27,7 +27,21 @@ export function sampleSpan(
   // work; prefer the hook if so
   let sampleRate;
   if (typeof options.tracesSampler === 'function') {
-    sampleRate = options.tracesSampler(samplingContext);
+    sampleRate = options.tracesSampler({
+      ...samplingContext,
+      inheritOrSampleWith(fallbackSampleRate) {
+        if (typeof samplingContext.parentSampleRate === 'number') {
+          return samplingContext.parentSampleRate;
+        }
+
+        // Fallback if parent sample rate is not on the incoming trace (e.g. if there is no baggage)
+        if (typeof samplingContext.parentSampled === 'boolean') {
+          return 1;
+        }
+
+        return fallbackSampleRate;
+      },
+    });
     localSampleRateWasApplied = true;
   } else if (samplingContext.parentSampled !== undefined) {
     sampleRate = samplingContext.parentSampled;

packages/core/src/types-hoist/samplingcontext.ts

@@ -41,4 +41,9 @@ export interface SamplingContext extends CustomSamplingContext {
 
   /** Initial attributes that have been passed to the span being sampled. */
   attributes?: SpanAttributes;
+
+  /**
+   * Returns a sample rate value that matches the sampling decision from the incoming trace, or falls back to the provided `fallbackSampleRate`.
+   */
+  inheritOrSampleWith: (fallbackSampleRate: number) => number;
 }