only use scope transaction name if event doesn't already have a transaction

Author: lobsterkatie

packages/hub/src/scope.ts

@@ -465,7 +465,11 @@ export class Scope implements ScopeInterface {
       event.level = this._level;
     }
     if (this._transactionName) {
-      event.transaction = this._transactionName;
+      // This runs before any event processors, so the only way that the event would already have a `transaction` value
+      // at this point is if either a) it's a transaction (they have a `transaction` value - their name - from the
+      // get-go, which we take great pains to ensure is as high-quality as possible), or b) it's a custom event in which
+      // the user has set the `transaction` value (and in that case we should respect that).
+      event.transaction = event.transaction || this._transactionName;
     }
 
     // We want to set the trace context for normal events only if there isn't already

packages/hub/test/scope.test.ts

@@ -302,14 +302,14 @@ describe('Scope', () => {
       });
     });
 
-    test('scope transaction should have priority over event transaction', async () => {
+    test("scope transaction shouldn't overwrite existing event transaction", async () => {
       expect.assertions(1);
       const scope = new Scope();
       scope.setTransactionName('/abc');
       const event: Event = {};
       event.transaction = '/cdf';
       return scope.applyToEvent(event).then(processedEvent => {
-        expect(processedEvent!.transaction).toEqual('/abc');
+        expect(processedEvent!.transaction).toEqual('/cdf');
       });
     });