Augment gaeinfo servlet with metadata server access

[ludoch]

In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/java-docs-samples/blob/master/appengine-java8/gaeinfo/src/main/java/com/example/appengine/standard/GAEInfoServlet.java

Internal doc in b/62589866
Would be nice to display all these entries in gaeinfo

[lesv]

Supported paths:

// /computeMetadata/v1/project/numeric-project-id  (pending)
// /computeMetadata/v1/project/project-id
// /computeMetadata/v1/instance/zone
// /computeMetadata/v1/instance/service-accounts/default/aliases
// /computeMetadata/v1/instance/service-accounts/default/email
// /computeMetadata/v1/instance/service-accounts/default/scopes
// /computeMetadata/v1/instance/service-accounts/default/token
// /computeMetadata/v1/instance/service-accounts/{account}/aliases
// /computeMetadata/v1/instance/service-accounts/{account}/email
// /computeMetadata/v1/instance/service-accounts/{account}/scopes
// /computeMetadata/v1/instance/service-accounts/{account}/token
// /computeMetadata/v1beta1/project/numeric-project-id
// /computeMetadata/v1beta1/project/project-id
// /computeMetadata/v1beta1/instance/zone  (pending)
// /computeMetadata/v1beta1/instance/service-accounts/default/aliases
// /computeMetadata/v1beta1/instance/service-accounts/default/email
// /computeMetadata/v1beta1/instance/service-accounts/default/scopes
// /computeMetadata/v1beta1/instance/service-accounts/default/token
// /computeMetadata/v1beta1/instance/service-accounts/{account}/aliases
// /computeMetadata/v1beta1/instance/service-accounts/{account}/email
// /computeMetadata/v1beta1/instance/service-accounts/{account}/scopes
// /computeMetadata/v1beta1/instance/service-accounts/{account}/token