test(compute): document rationale for credential refresh in observability tests

Author: diegomarquezp

java-compute/google-cloud-compute/src/test/java/com/google/cloud/compute/v1/integration/ITComputeGoldenSignals.java

@@ -113,6 +113,9 @@ public void setUp() throws Exception {
     testRunId = generateRandomHexString(8);
 
     GoogleCredentials credentials = GoogleCredentials.getApplicationDefault();
+    // We explicitly refresh the credentials here to guarantee a valid token is extracted.
+    // The standard OTLP exporter does not automatically manage OAuth token lifecycles 
+    // when they are passed as static header strings.
     credentials.refreshIfExpired();
     String token = credentials.getAccessToken().getTokenValue();