GCE_METADATA_HOST env does not work with IPv6 addresses

[marqc]

Steps to reproduce

1. Set custom GCE_METADATA_HOST env variable to IPv6 address, i.e. ::1
2. Observe application is trying to call invalid URL http://::1 instead of expected http://[::1]

google-api-java-client/google-api-client/src/main/java/com/google/api/client/googleapis/auth/oauth2/OAuth2Utils.java

Line 108 in 7ccb2fc

return "http://" + metadataServerAddress;