fix(node): Avoid setting sentry-trace header multiple times

If, for whatever reason, `propagator.inject()` is called multiple times, we do not want to add multiple `sentry-trace` headers. In this case, the first one wins.

packages/opentelemetry/src/propagator.ts

@@ -87,7 +87,9 @@ export class SentryPropagator extends W3CBaggagePropagator {
     }
 
     // We also want to avoid setting the default OTEL trace ID, if we get that for whatever reason
-    if (traceId && traceId !== INVALID_TRACEID) {
+    // If a sentry-trace header already exists, we do nothing
+    const existingSentryTrace = getExistingSentryTrace(carrier);
+    if (traceId && traceId !== INVALID_TRACEID && !existingSentryTrace) {
       setter.set(carrier, SENTRY_TRACE_HEADER, generateSentryTraceHeader(traceId, spanId, sampled));
     }
 
@@ -264,6 +266,16 @@ function getExistingBaggage(carrier: unknown): string | undefined {
   }
 }
 
+/** Try to get the existing sentry-trace header. */
+function getExistingSentryTrace(carrier: unknown): string | undefined {
+  try {
+    const sentryTrace = (carrier as Record<string, string | string[]>)[SENTRY_TRACE_HEADER];
+    return Array.isArray(sentryTrace) ? sentryTrace.join(',') : sentryTrace;
+  } catch {
+    return undefined;
+  }
+}
+
 /**
  * It is pretty tricky to get access to the outgoing request URL of a request in the propagator.
  * As we only have access to the context of the span to be sent and the carrier (=headers),