fix(core): Ensure standalone spans are not sent if SDK is disabled (#14088)

Change the sending logic for standalone spans to to use the client's 
`sendEnvelope` method which we generally use to send envelopes 
(sessions, client reports, checkins, metrics (RIP), and also events). 
This has a minor implication: We will now also emit a `beforeEnvelope` 
client hook event for sending standalone spans.

fixes #14082

Author: Lms24

dev-packages/browser-integration-tests/suites/public-api/startSpan/standalone-sdk-disabled/init.js

@@ -0,0 +1,18 @@
+import * as Sentry from '@sentry/browser';
+
+window.Sentry = Sentry;
+
+window.fetchCallCount = 0;
+window.spanEnded = false;
+
+const originalWindowFetch = window.fetch;
+window.fetch = (...args) => {
+  window.fetchCallCount++;
+  return originalWindowFetch(...args);
+};
+
+Sentry.init({
+  dsn: 'https://public@dsn.ingest.sentry.io/1337',
+  tracesSampleRate: 1.0,
+  enabled: false,
+});

dev-packages/browser-integration-tests/suites/public-api/startSpan/standalone-sdk-disabled/subject.js

@@ -0,0 +1,3 @@
+Sentry.startSpan({ name: 'standalone_segment_span', experimental: { standalone: true } }, () => {});
+
+window.spanEnded = true;

dev-packages/browser-integration-tests/suites/public-api/startSpan/standalone-sdk-disabled/test.ts

@@ -0,0 +1,22 @@
+import { expect } from '@playwright/test';
+
+import { sentryTest } from '../../../../utils/fixtures';
+import { shouldSkipTracingTest } from '../../../../utils/helpers';
+
+sentryTest("doesn't send a standalone span envelope if SDK is disabled", async ({ getLocalTestPath, page }) => {
+  if (shouldSkipTracingTest()) {
+    sentryTest.skip();
+  }
+
+  const url = await getLocalTestPath({ testDir: __dirname });
+  await page.goto(url);
+
+  // @ts-expect-error this exists in the test init/subject
+  await page.waitForFunction(() => !!window.spanEnded);
+  await page.waitForTimeout(2000);
+
+  // @ts-expect-error this exists in the test init
+  const fetchCallCount = await page.evaluate(() => window.fetchCallCount);
+  // We expect no fetch calls because the SDK is disabled
+  expect(fetchCallCount).toBe(0);
+});

packages/core/src/baseclient.ts

@@ -565,7 +565,7 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
 
     if (this._isEnabled() && this._transport) {
       return this._transport.send(envelope).then(null, reason => {
-        DEBUG_BUILD && logger.error('Error while sending event:', reason);
+        DEBUG_BUILD && logger.error('Error while sending envelope:', reason);
         return reason;
       });
     }

packages/core/src/tracing/sentrySpan.ts

@@ -429,10 +429,7 @@ function sendSpanEnvelope(envelope: SpanEnvelope): void {
     return;
   }
 
-  const transport = client.getTransport();
-  if (transport) {
-    transport.send(envelope).then(null, reason => {
-      DEBUG_BUILD && logger.error('Error while sending span:', reason);
-    });
-  }
+  // sendEnvelope should not throw
+  // eslint-disable-next-line @typescript-eslint/no-floating-promises
+  client.sendEnvelope(envelope);
 }