title: Auth for connecting to Bigtable from a compute instance
body: <p>I have an app connecting to Google Bigtable using the <code>com.google.cloud.bigtable.hbase1_2.BigtableConnection</code> class. This is running inside a compute engine VM which has the default scopes set plus the <code>https://www.googleapis.com/auth/bigtable.data</code> scope (required for reading/writing to Bigtable). The connection works fine when the property <code>google.bigtable.auth.json.keyfile</code> is set to the appropriate auth file. </p>
  
  <p>However, I want to avoid setting this property and just use the scopes set for the instance, so that the key file does not have to be kept on the instance. This does not seem to work. The error message is</p>
  
  <pre><code>Caused by: com.google.bigtable.repackaged.io.grpc.StatusRuntimeException: PERMISSION_DENIED: Request had insufficient authentication scopes.
      at com.google.bigtable.repackaged.io.grpc.stub.ClientCalls.toStatusRuntimeException(ClientCalls.java:230)
      at com.google.bigtable.repackaged.io.grpc.stub.ClientCalls.getUnchecked(ClientCalls.java:211)
      at com.google.bigtable.repackaged.io.grpc.stub.ClientCalls.blockingUnaryCall(ClientCalls.java:144)
      at com.google.bigtable.admin.v2.BigtableTableAdminGrpc$BigtableTableAdminBlockingStub.listTables(BigtableTableAdminGrpc.java:371)
      at com.google.cloud.bigtable.grpc.BigtableTableAdminGrpcClient.listTables(BigtableTableAdminGrpcClient.java:52)
      at org.apache.hadoop.hbase.client.AbstractBigtableAdmin.requestTableList(AbstractBigtableAdmin.java:258)
      ... 8 more
  </code></pre>
  
  <p>Is there any other way of doing this or is there a configuration property I'm missing?</p>