Update static GCP metadata URL

[nicktrav]

When running Istio on Google Kubernetes Enginge (GKE), the Envoy sidecar
proxy will fail to route any requests made to the metadata server via
the metadata hostname (requests will 404). Requests made to the fully
qualified metadata.google.internal hostname can be routed.

Update the constant for the metadata host in GcpMetadataConfig to use
the FQDN.

[nicktrav]

This follows what was done in googleapis/google-cloud-java#4278 (fixing the same Istio on GKE problem raised in googleapis/google-cloud-java/pull/4269). Looks like GcpMetadataConfig is a port from the code in that repo, so I just ported the change over here.

[songy23]

Will this be compatible with non-GKE environment? e.g GCE instances

[nicktrav]

I can't really speak for GCP, but it seems like GCE knows about the FQDN:

nick.travers@nickt-test:~$ host metadata
metadata.google.internal has address 169.254.169.254
nick.travers@nickt-test:~$ host metadata.google.internal
metadata.google.internal has address 169.254.169.254

nick.travers@nickt-test:~$ curl -i http://metadata
HTTP/1.1 200 OK
Metadata-Flavor: Google
Content-Type: application/text
Date: Sun, 21 Apr 2019 17:23:01 GMT
Server: Metadata Server for VM
Content-Length: 22
X-XSS-Protection: 1; mode=block
X-Frame-Options: SAMEORIGIN

0.1/
computeMetadata/
nick.travers@nickt-test:~$ curl -i http://metadata.google.internal
HTTP/1.1 200 OK
Metadata-Flavor: Google
Content-Type: application/text
Date: Sun, 21 Apr 2019 17:23:07 GMT
Server: Metadata Server for VM
Content-Length: 22
X-XSS-Protection: 1; mode=block
X-Frame-Options: SAMEORIGIN

0.1/
computeMetadata/

The fact that it was merged into Google's core Java library gives me more confidence this is ok:
googleapis/google-cloud-java#4278

[songy23]

LGTM, thanks for making this change.