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: pubsub.go
    path of file: ./repos/google-cloud-go/pubsub
    
    the code of the file until where you have to start completion: // Copyright 2014 Google LLC
//
// 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 pubsub // import "cloud.google.com/go/pubsub"

import (
	"context"
	"errors"
	"fmt"
	"os"
	"reflect"
	"runtime"
	"strings"
	"time"

	"cloud.google.com/go/internal/detect"
	vkit "cloud.google.com/go/pubsub/apiv1"
	"cloud.google.com/go/pubsub/internal"
	gax "github.com/googleapis/gax-go/v2"
	"google.golang.org/api/option"
	"google.golang.org/grpc"
	"google.golang.org/grpc/keepalive"
)

const (
	// ScopePubSub grants permissions to view and manage Pub/Sub
	// topics and subscriptions.
	ScopePubSub = "https://www.googleapis.com/auth/pubsub"

	// ScopeCloudPlatform grants permissions to view and manage your data
	// across Google Cloud Platform services.
	ScopeCloudPlatform = "https://www.googleapis.com/auth/cloud-platform"
)

// Client is a Google Pub/Sub client scoped to a single project.
//
// Clients should be reused rather than being created as needed.
// A Client may be shared by multiple goroutines.
type Client struct {
	projectID string
	pubc      *vkit.PublisherClient
	subc      *vk
    
func NewClientWithConfig(ctx context.Context, projectID string, config *ClientConfig, opts ...option.ClientOption) (c *Client, err error) {
	if projectID == "" {
		return nil, ErrEmptyProjectID
	}
	var o []option.ClientOption
	// Environment variables for gcloud emulator:
	// https://cloud.google.com/sdk/gcloud/reference/beta/emulators/pubsub/
	if addr := os.Getenv("PUBSUB_EMULATOR_HOST"); addr != "" {
		conn, err := grpc.Dial(addr, grpc.WithInsecure())
		if err != nil {
			return nil, fmt.Errorf("grpc.Dial: %w", err)
		}
		o = []option.ClientOption{option.WithGRPCConn(conn)}
		o = append(o, option.WithTelemetryDisabled())
	} else {
		numConns := runtime.GOMAXPROCS(0)
		if numConns > 4 {
			numConns = 4
		}
		o = []option.ClientOption{
			// Create multiple connections to increase throughput.
			option.WithGRPCConnectionPool(numConns),
			option.WithGRPCDialOption(grpc.WithKeepaliveParams(keepalive.ClientParameters{
				Time: 5 * time.Minute,
			})),
		}
	}
	o = append(o, opts...)
	pubc, err := vkit.NewPublisherClient(ctx, o...)
	if err != nil {
		return nil, fmt.Errorf("pubsub(publisher): %w", err)
	}
	subc, err := vkit.NewSubscriberClient(ctx, o...)
	if err != nil {
		return nil, fmt.Errorf("pubsub(subscriber): %w", err)
	}
	if config != nil {
		pubc.CallOptions = mergePublisherCallOptions(pubc.CallOptions, config.PublisherCallOptions)
		subc.CallOptions = mergeSubscriberCallOptions(subc.CallOptions, config.SubscriberCallOptions)
	}
	pubc.SetGoogleClientInfo("gccl", internal.Version)
	subc.SetGoogleClientInfo("gccl", internal.Version)

	// Handle project autodetection.
	projectID, err = detect.ProjectID(ctx, projectID, "", opts...)
	if err != nil {
		return nil, err
	}

	return &Client{
		projectID: projectID,
		pubc:      pubc,
		subc:      subc,
	}, nil
}