feat: add logging at FINE level for each step of ADC (googleapis#435)

* feat: add logging at FINE level for each step of Application Default Credentials

* chore: fix formatting

* fix: standardize the debug message format

oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java

@@ -253,7 +253,7 @@ static boolean runningOnComputeEngine(
             e);
       }
     }
-    LOGGER.log(Level.INFO, "Failed to detect whether we are running on Google Compute Engine.");
+    LOGGER.log(Level.FINE, "Failed to detect whether we are running on Google Compute Engine.");
     return false;
   }
 

oauth2_http/java/com/google/auth/oauth2/DefaultCredentialsProvider.java

@@ -145,6 +145,9 @@ private final GoogleCredentials getDefaultCredentialsUnsynchronized(
     GoogleCredentials credentials = null;
     String credentialsPath = getEnv(CREDENTIAL_ENV_VAR);
     if (credentialsPath != null && credentialsPath.length() > 0) {
+      LOGGER.log(
+          Level.FINE,
+          String.format("Attempting to load credentials from file: %s", credentialsPath));
       InputStream credentialsStream = null;
       try {
         File credentialsFile = new File(credentialsPath);
@@ -178,6 +181,11 @@ private final GoogleCredentials getDefaultCredentialsUnsynchronized(
       InputStream credentialsStream = null;
       try {
         if (isFile(wellKnownFileLocation)) {
+          LOGGER.log(
+              Level.FINE,
+              String.format(
+                  "Attempting to load credentials from well known file: %s",
+                  wellKnownFileLocation.getCanonicalPath()));
           credentialsStream = readStream(wellKnownFileLocation);
           credentials = GoogleCredentials.fromStream(credentialsStream, transportFactory);
         }
@@ -198,17 +206,20 @@ private final GoogleCredentials getDefaultCredentialsUnsynchronized(
 
     // Then try GAE 7 standard environment
     if (credentials == null && isOnGAEStandard7() && !skipAppEngineCredentialsCheck()) {
+      LOGGER.log(Level.FINE, "Attempting to load credentials from GAE 7 Standard");
       credentials = tryGetAppEngineCredential();
     }
 
     // Then try Cloud Shell.  This must be done BEFORE checking
     // Compute Engine, as Cloud Shell runs on GCE VMs.
     if (credentials == null) {
+      LOGGER.log(Level.FINE, "Attempting to load credentials from Cloud Shell");
       credentials = tryGetCloudShellCredentials();
     }
 
     // Then try Compute Engine and GAE 8 standard environment
     if (credentials == null) {
+      LOGGER.log(Level.FINE, "Attempting to load credentials from GCE");
       credentials = tryGetComputeCredentials(transportFactory);
     }