Fix InstantiatingGrpcChannelProvider's channel pool to play nicely with DirectPath

gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java

@@ -42,6 +42,7 @@
 import com.google.auth.oauth2.ComputeEngineCredentials;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import io.grpc.ManagedChannel;
 import io.grpc.ManagedChannelBuilder;
 import io.grpc.alts.ComputeEngineChannelBuilder;
@@ -238,6 +239,21 @@ private ManagedChannel createSingleChannel() throws IOException {
       // Will be overridden by user defined values if any.
       builder.keepAliveTime(DIRECT_PATH_KEEP_ALIVE_TIME_SECONDS, TimeUnit.SECONDS);
       builder.keepAliveTimeout(DIRECT_PATH_KEEP_ALIVE_TIMEOUT_SECONDS, TimeUnit.SECONDS);
+
+      // When channel pooling is enabled, force the pick_first grpclb strategy.
+      // This is necessary to avoid the multiplicative effect of creating channel pool with
+      // `poolSize` number of  `ManagedChannel`s, each with a `subSetting` number of number of subchannels.
+      if (poolSize > 1) {
+        builder.defaultServiceConfig(
+            ImmutableMap.of(
+                "loadBalancingConfig",
+                ImmutableList.of(
+                    ImmutableMap.of(
+                        "grpclb",
+                        ImmutableMap.of(
+                            "childPolicy",
+                            ImmutableList.of(ImmutableMap.of("pick_first", ImmutableMap.of())))))));
+      }
     } else {
       builder = ManagedChannelBuilder.forAddress(serviceAddress, port);
     }