Code:play 

package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// Use Google Application Default Credentials to authorize and authenticate the client.
	// More information about Application Default Credentials and how to enable is at
	// https://developers.google.com/identity/protocols/application-default-credentials.
	//
	// This is the recommended way of authorizing and authenticating.
	//
	// Note: The example uses the datastore client, but the same steps apply to
	// the other client libraries underneath this package.
	client, err := datastore.NewClient(ctx, "project-id")
	if err != nil {
		// TODO: handle error.
	}
	// Use the client.
	_ = client
}

Code:play 

package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
	"google.golang.org/api/option"
)

func main() {
	// Warning: The better way to use service accounts is to set GOOGLE_APPLICATION_CREDENTIALS
	// and use the Application Default Credentials.
	ctx := context.Background()
	// Use a JSON key file associated with a Google service account to
	// authenticate and authorize.
	// Go to https://console.developers.google.com/permissions/serviceaccounts to create
	// and download a service account key for your project.
	//
	// Note: The example uses the datastore client, but the same steps apply to
	// the other client libraries underneath this package.
	client, err := datastore.NewClient(ctx,
		"project-id",
		option.WithServiceAccountFile("/path/to/service-account-key.json"))
	if err != nil {
		// TODO: handle error.
	}
	// Use the client.
	_ = client
}