add additional error handling test + some comments

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/EnhancedBigtableStub.java

@@ -133,6 +133,10 @@ public static EnhancedBigtableStubSettings finalizeSettings(
       throws IOException {
     EnhancedBigtableStubSettings.Builder builder = settings.toBuilder();
 
+    // TODO: this implementation is on the cusp of unwieldy, if we end up adding more features
+    // consider splitting it up by feature.
+
+    // Inject channel priming
     if (settings.isRefreshingChannel()) {
       // Fix the credentials so that they can be shared
       Credentials credentials = null;

google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/stub/BigtableChannelPrimerTest.java

@@ -50,6 +50,8 @@
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
+import org.mockito.Mockito;
+import org.mockito.internal.stubbing.answers.ThrowsException;
 
 @RunWith(JUnit4.class)
 public class BigtableChannelPrimerTest {
@@ -158,6 +160,27 @@ public ReadRowsResponse apply(ReadRowsRequest req) {
     }
   }
 
+  @Test
+  public void testErrorsAreLoggedForBasic() {
+    BigtableChannelPrimer basicPrimer =
+        BigtableChannelPrimer.create(
+            OAuth2Credentials.create(new AccessToken(TOKEN_VALUE, null)),
+            "fake-project",
+            "fake-instance",
+            "fake-app-profile",
+            ImmutableList.<String>of());
+
+    ManagedChannel channel =
+        Mockito.mock(
+            ManagedChannel.class, new ThrowsException(new UnsupportedOperationException()));
+    primer.primeChannel(channel);
+
+    assertThat(logHandler.logs).hasSize(1);
+    for (LogRecord log : logHandler.logs) {
+      assertThat(log.getMessage()).contains("Unexpected");
+    }
+  }
+
   private static class MetadataInterceptor implements ServerInterceptor {
     ConcurrentLinkedQueue<Metadata> metadataList = new ConcurrentLinkedQueue<>();