fix(gax-grpc): add pick_first fallback to direct path service config

Author: westarle

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

@@ -1437,7 +1437,8 @@ public ApiFunction<ManagedChannelBuilder, ManagedChannelBuilder> getChannelConfi
     ImmutableMap<String, Object> grpcLbPolicy =
         ImmutableMap.<String, Object>of("grpclb", childPolicy);
 
-    return ImmutableMap.<String, Object>of("loadBalancingConfig", ImmutableList.of(grpcLbPolicy));
+    return ImmutableMap.<String, Object>of(
+        "loadBalancingConfig", ImmutableList.of(grpcLbPolicy, pickFirstStrategy));
   }
 
   private static void validateEndpoint(String endpoint) {

gax-java/gax-grpc/src/test/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProviderTest.java

@@ -547,13 +547,14 @@ void testWithDefaultDirectPathServiceConfig() {
     ImmutableMap<String, ?> defaultServiceConfig = provider.directPathServiceConfig;
 
     List<Map<String, ?>> lbConfigs = getAsObjectList(defaultServiceConfig, "loadBalancingConfig");
-    assertThat(lbConfigs).hasSize(1);
+    assertThat(lbConfigs).hasSize(2);
     Map<String, ?> lbConfig = lbConfigs.get(0);
     Map<String, ?> grpclb = getAsObject(lbConfig, "grpclb");
     List<Map<String, ?>> childPolicies = getAsObjectList(grpclb, "childPolicy");
     assertThat(childPolicies).hasSize(1);
     Map<String, ?> childPolicy = childPolicies.get(0);
     assertThat(childPolicy.keySet()).containsExactly("pick_first");
+    assertThat(lbConfigs.get(1).keySet()).containsExactly("pick_first");
   }
 
   @Nullable