transport/grpc: use "pick_first" as child policy

For DirectPath, we need to specify the service config to use "pick_first" as the child policy for grpclb,
so when channel pool is used (which is necessary for CloudPath), the client won't create too many sub connections.

Change-Id: I2ea8c544a15bd516c4deffb436f129130bbfe2fc
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/50575
Reviewed-by: Chris Broadfoot <cbro@google.com>

transport/grpc/dial.go

diff --git a/transport/grpc/dial.go b/transport/grpc/dial.go
index 1d2c9df..adad182 100644
--- a/transport/grpc/dial.go
+++ b/transport/grpc/dial.go
@@ -135,6 +135,11 @@
 				grpc.WithCredentialsBundle(
 					grpcgoogle.NewComputeEngineCredentials(),
 				),
+				// For now all DirectPath go clients will be using the following lb config, but in future
+				// when different services need different configs, then we should change this to a
+				// per-service config.
+				grpc.WithDisableServiceConfig(),
+				grpc.WithDefaultServiceConfig(`{"loadBalancingConfig":[{"grpclb":{"childPolicy":[{"pick_first":{}}]}}]}`),
 			}
 			// TODO(cbro): add support for system parameters (quota project, request reason) via chained interceptor.
 		} else {