fix(core): Ensure standalone spans respect sampled flag (#12533)

`sendSpanEnvelope` immediately sends
spans without checking for `this._sampled`, which means you can't use
`tracesSampler` to filter them. This changes that!

We also record client outcomes for `sample_rate` with spans.

packages/core/src/tracing/sentrySpan.ts

@@ -261,7 +261,15 @@ export class SentrySpan implements Span {
 
     // if this is a standalone span, we send it immediately
     if (this._isStandaloneSpan) {
-      sendSpanEnvelope(createSpanEnvelope([this], client));
+      if (this._sampled) {
+        sendSpanEnvelope(createSpanEnvelope([this], client));
+      } else {
+        DEBUG_BUILD &&
+          logger.log('[Tracing] Discarding standalone span because its trace was not chosen to be sampled.');
+        if (client) {
+          client.recordDroppedEvent('sample_rate', 'span');
+        }
+      }
       return;
     }
 

packages/core/test/lib/tracing/sentrySpan.test.ts

@@ -91,6 +91,39 @@ describe('SentrySpan', () => {
       expect(spanToJSON(span).timestamp).toBeGreaterThan(1);
     });
 
+    test('uses sampled config for standalone span', () => {
+      const client = new TestClient(
+        getDefaultTestClientOptions({
+          dsn: 'https://username@domain/123',
+          enableSend: true,
+        }),
+      );
+      setCurrentClient(client);
+
+      // @ts-expect-error Accessing private transport API
+      const mockSend = jest.spyOn(client._transport, 'send');
+
+      const notSampledSpan = new SentrySpan({
+        name: 'not-sampled',
+        isStandalone: true,
+        startTimestamp: 1,
+        endTimestamp: 2,
+        sampled: false,
+      });
+      notSampledSpan.end();
+      expect(mockSend).not.toHaveBeenCalled();
+
+      const sampledSpan = new SentrySpan({
+        name: 'is-sampled',
+        isStandalone: true,
+        startTimestamp: 1,
+        endTimestamp: 2,
+        sampled: true,
+      });
+      sampledSpan.end();
+      expect(mockSend).toHaveBeenCalledTimes(1);
+    });
+
     test('sends the span if `beforeSendSpan` does not modify the span ', () => {
       const beforeSendSpan = jest.fn(span => span);
       const client = new TestClient(