test(core): Show that unsampled spans are counted even if they would not be sent

Unsampled spans record their outcome when they start, so spans a sampled
transaction would never contain (e.g. unfinished children) are counted too.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: Lms24

dev-packages/node-integration-tests/suites/tracing/sampling-static/test.ts

@@ -9,7 +9,7 @@ describe('negative sampling (static)', () => {
   createEsmAndCjsTests(__dirname, 'server.mjs', 'instrument.mjs', (createRunner, test) => {
     test('records sample_rate outcomes for the transaction and all of its spans', async () => {
       // `/health` and `/ok` go through the same middleware, so the `/ok` transaction tells us how many
-      // spans the dropped `/health` transaction woudl have had. The count differs per runtime (e.g. Bun creates
+      // spans the dropped `/health` transaction would have had. The count differs per runtime (e.g. Bun creates
       // no Express spans), so we derive it instead of hardcoding it.
       let okSpanCount: number | undefined;
       let droppedSpanCount: number | undefined;

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

@@ -2828,6 +2828,34 @@ describe('ignoreSpans (core path, streaming)', () => {
     expect(spyOnDroppedEvent).toHaveBeenCalledWith('sample_rate', 'span');
   });
 
+  // Unsampled spans are counted when they start, so this also counts spans a sampled
+  // transaction would never contain (here: a child that never ends).
+  it('records sample_rate outcomes for unsampled spans that a sampled transaction would not contain', () => {
+    const runScenario = (tracesSampleRate: number): ReturnType<typeof vi.spyOn> => {
+      client = new TestClient(getDefaultTestClientOptions({ tracesSampleRate }));
+      setCurrentClient(client);
+      client.init();
+      const spyOnDroppedEvent = vi.spyOn(client, 'recordDroppedEvent');
+
+      startSpan({ name: 'GET /foo' }, () => {
+        startInactiveSpan({ name: 'never ended' });
+      });
+
+      return spyOnDroppedEvent;
+    };
+
+    const sampledSpy = vi.spyOn(TestClient.prototype, 'sendEvent');
+    runScenario(1);
+    expect(sampledSpy).toHaveBeenCalledWith(expect.objectContaining({ spans: [] }), expect.anything());
+    sampledSpy.mockRestore();
+
+    const unsampledDroppedEventSpy = runScenario(0);
+    expect(unsampledDroppedEventSpy).toHaveBeenCalledTimes(3);
+    expect(unsampledDroppedEventSpy).toHaveBeenNthCalledWith(1, 'sample_rate', 'transaction');
+    expect(unsampledDroppedEventSpy).toHaveBeenNthCalledWith(2, 'sample_rate', 'span');
+    expect(unsampledDroppedEventSpy).toHaveBeenNthCalledWith(3, 'sample_rate', 'span');
+  });
+
   it('records only one ignored outcome for directly ignored child span', () => {
     const options = getDefaultTestClientOptions({
       tracesSampleRate: 1,