never overrule explicitly-provided sampling decision

Author: lobsterkatie

packages/tracing/src/hubextensions.ts

@@ -62,6 +62,11 @@ function sample<T extends Transaction>(hub: Hub, transaction: T, samplingContext
     return transaction;
   }
 
+  // if the user has forced a sampling decision by passing a `sampled` value in their transaction context, go with that
+  if (transaction.sampled !== undefined) {
+    return transaction;
+  }
+
   // we would have bailed already if neither `tracesSampler` nor `tracesSampleRate` were defined, so one of these should
   // work; prefer the hook if so
   const sampleRate =

packages/tracing/test/hub.test.ts

@@ -193,6 +193,14 @@ describe('Hub', () => {
         expect(transaction.sampled).toBe(false);
       });
 
+      it('should not try to override sampling decision provided in transaction context', () => {
+        // setting tracesSampleRate to 1 means that without the override, the sampling decision should be true
+        const hub = new Hub(new BrowserClient({ tracesSampleRate: 1 }));
+        const transaction = hub.startTransaction({ name: 'dogpark', sampled: false });
+
+        expect(transaction.sampled).toBe(false);
+      });
+
       it('should not sample transactions when tracesSampleRate is 0', () => {
         const hub = new Hub(new BrowserClient({ tracesSampleRate: 0 }));
         const transaction = hub.startTransaction({ name: 'dogpark' });