Option to suppress end user credentials warning. (#207)

* Option to suppress end user credentials warning.

Defining a new environment variable SUPPRESS_GCLOUD_CREDS_WARNING.
Setting this to true suppresses the end user credentials warning.
Fixes #193

* reordering checks to respond to comment

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

@@ -87,6 +87,7 @@ class DefaultCredentialsProvider {
       + "SDK, you might receive a \"quota exceeded\" or \"API not enabled\" error. For "
       + "more information about service accounts, see "
       + "https://cloud.google.com/docs/authentication/.";
+  public static final String SUPPRESS_GCLOUD_CREDS_WARNING_ENV_VAR = "SUPPRESS_GCLOUD_CREDS_WARNING";
 
   // These variables should only be accessed inside a synchronized block
   private GoogleCredentials cachedCredentials = null;
@@ -211,8 +212,9 @@ private final GoogleCredentials getDefaultCredentialsUnsynchronized(
   }
 
   private void warnAboutProblematicCredentials(GoogleCredentials credentials) {
-    if (credentials instanceof UserCredentials &&
-        ((UserCredentials)credentials).getClientId().equals(CLOUDSDK_CLIENT_ID)) {
+    if (credentials instanceof UserCredentials
+        && ((UserCredentials) credentials).getClientId().equals(CLOUDSDK_CLIENT_ID)
+        && !Boolean.parseBoolean(getEnv(SUPPRESS_GCLOUD_CREDS_WARNING_ENV_VAR))) {
       LOGGER.log(Level.WARNING, CLOUDSDK_CREDENTIALS_WARNING);
     }
   }

oauth2_http/javatests/com/google/auth/oauth2/DefaultCredentialsProviderTest.java

@@ -34,6 +34,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
@@ -465,6 +466,19 @@ public void flush() {}
 
   @Test
   public void getDefaultCredentials_wellKnownFile_logsGcloudWarning() throws IOException {
+    LogRecord message = getCredentialsAndReturnLogMessage(false);
+    assertNotNull(message);
+    assertEquals(Level.WARNING, message.getLevel());
+    assertTrue(message.getMessage().contains("end user credentials from Google Cloud SDK"));
+  }
+
+  @Test
+  public void getDefaultCredentials_wellKnownFile_suppressGcloudWarning() throws IOException {
+    LogRecord message = getCredentialsAndReturnLogMessage(true);
+    assertNull(message);
+  }
+
+  private LogRecord getCredentialsAndReturnLogMessage(boolean suppressWarning) throws IOException {
     Logger logger = Logger.getLogger(DefaultCredentialsProvider.class.getName());
     LogHandler handler = new LogHandler();
     logger.addHandler(handler);
@@ -477,16 +491,13 @@ public void getDefaultCredentials_wellKnownFile_logsGcloudWarning() throws IOExc
     File wellKnownFile =
         new File(cloudConfigDir, DefaultCredentialsProvider.WELL_KNOWN_CREDENTIALS_FILE);
     TestDefaultCredentialsProvider testProvider = new TestDefaultCredentialsProvider();
+    testProvider.setEnv(DefaultCredentialsProvider.SUPPRESS_GCLOUD_CREDS_WARNING_ENV_VAR, Boolean.toString(suppressWarning));
     testProvider.setProperty("os.name", "linux");
     testProvider.setProperty("user.home", homeDir.getAbsolutePath());
     testProvider.addFile(wellKnownFile.getAbsolutePath(), userStream);
-
     testUserProvidesToken(
         testProvider, GCLOUDSDK_CLIENT_ID, USER_CLIENT_SECRET, REFRESH_TOKEN);
-    LogRecord message = handler.getRecord();
-    assertNotNull(message);
-    assertEquals(Level.WARNING, message.getLevel());
-    assertTrue(message.getMessage().contains("end user credentials from Google Cloud SDK"));
+    return handler.getRecord();
   }
 
   private static File getTempDirectory() {