You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: pickfirst.go
    path of file: ./repos/moby/vendor/google.golang.org/grpc
    
    the code of the file until where you have to start completion: /*
 *
 * Copyright 2017 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

package grpc

import (
	"encoding/json"
	"errors"
	"fmt"

	"google.golang.org/grpc/balancer"
	"google.golang.o
    
func (*pickfirstBuilder) ParseConfig(js json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
	if !envconfig.PickFirstLBConfig {
		// Prior to supporting loadbalancing configuration, the pick_first LB
		// policy did not implement the balancer.ConfigParser interface. This
		// meant that if a non-empty configuration was passed to it, the service
		// config unmarshaling code would throw a warning log, but would
		// continue using the pick_first LB policy. The code below ensures the
		// same behavior is retained if the env var is not set.
		if string(js) != "{}" {
			logger.Warningf("Ignoring non-empty balancer configuration %q for the pick_first LB policy", string(js))
		}
		return nil, nil
	}

	var cfg pfConfig
	if err := json.Unmarshal(js, &cfg); err != nil {
		return nil, fmt.Errorf("pickfirst: unable to unmarshal LB policy config: %s, error: %v", string(js), err)
	}
	return cfg, nil
}