perf(node): Short circuit flushing on Vercel only for Vercel

packages/node/src/integrations/http/SentryHttpInstrumentationBeforeOtel.ts

@@ -9,6 +9,17 @@ import { stealthWrap } from './utils';
 type Http = typeof http;
 type Https = typeof https;
 
+// The reason this "before OTEL" integration even exists is due to timing reasons. We need to be able to register a
+// `res.on('close')` handler **after** OTEL registers its own handler (which it uses to end spans), so that we can do
+// something (ie. flush) after OTEL has ended a span for a request. If you think about it like an onion:
+//
+// (Sentry after OTEL instrumentation
+//   (OTEL instrumentation
+//     (Sentry before OTEL instrumentation
+//       (orig HTTP request handler))))
+//
+// registering an instrumentation before OTEL allows us to do this for incoming requests.
+
 /**
  * A Sentry specific http instrumentation that is applied before the otel instrumentation.
  */

packages/node/src/integrations/http/index.ts

@@ -142,7 +142,17 @@ export const httpIntegration = defineIntegration((options: HttpOptions = {}) =>
   return {
     name: INTEGRATION_NAME,
     setupOnce() {
-      instrumentSentryHttpBeforeOtel();
+      // Below, we instrument the Node.js HTTP API three times. 2 times Sentry-specific, 1 time OTEL specific.
+      // Due to timing reasons, we sometimes need to apply Sentry instrumentation _before_ we apply the OTEL
+      // instrumentation (e.g. to flush on serverless platforms), and sometimes we need to apply Sentry instrumentation
+      // _after_ we apply OTEL instrumentation (e.g. for isolation scope handling and breadcrumbs).
+
+      // This is Sentry-specific instrumentation that is applied _before_ any OTEL instrumentation.
+      if (process.env.VERCEL) {
+        // Currently this instrumentation only does something when deployed on Vercel, so to save some overhead, we short circuit adding it here only for Vercel.
+        // If it's functionality is extended in the future, feel free to remove the if statement and this comment.
+        instrumentSentryHttpBeforeOtel();
+      }
 
       const instrumentSpans = _shouldInstrumentSpans(options, getClient<NodeClient>()?.getOptions());
 
@@ -152,9 +162,7 @@ export const httpIntegration = defineIntegration((options: HttpOptions = {}) =>
         instrumentOtelHttp(instrumentationConfig);
       }
 
-      // This is the Sentry-specific instrumentation that isolates requests & creates breadcrumbs
-      // Note that this _has_ to be wrapped after the OTEL instrumentation,
-      // otherwise the isolation will not work correctly
+      // This is Sentry-specific instrumentation that is applied _after_ any OTEL instrumentation.
       instrumentSentryHttp({
         ...options,
         // If spans are not instrumented, it means the HttpInstrumentation has not been added