Incorrect metadata endpoint in GAE standard sample

Issue found in the following file:
https://github.com/GoogleCloudPlatform/nodejs-docs-samples/blob/master/appengine/metadata/standard/server.js#L25

According to the currently documented list of App Engine metadata server endpoints, the endpoint used in this sample (/computeMetadata/v1/instance/network-interfaces) is not available in the App Engine standard environment.

[michaelawyu]

I will update this sample to use project-id instead external-ip :)

[michaelawyu]

@ericschmidtatwork PR sent: #617

I vaguely recall that this sample was updated to use project ID before...

[michaelawyu]

PR merged: #624