google/internal/externalaccount: Adding metadata verification

Change-Id: I4d664862b7b287131c1481b238ebd0875f7c233b
GitHub-Last-Rev: 74bcc33f5ed4863c740aaf09ad4ee3ac4366e8e1
GitHub-Pull-Request: golang/oauth2#608
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/449975
Run-TryBot: Cody Oss <codyoss@google.com>
Auto-Submit: Cody Oss <codyoss@google.com>
Reviewed-by: Leo Siracusa <leosiracusa@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

google/internal/externalaccount/basecredentials.go

@@ -213,6 +213,10 @@ func (c *Config) parse(ctx context.Context) (baseCredentialSource, error) {
 				awsCredSource.IMDSv2SessionTokenURL = c.CredentialSource.IMDSv2SessionTokenURL
 			}
 
+			if err := awsCredSource.validateMetadataServers(); err != nil {
+				return nil, err
+			}
+
 			return awsCredSource, nil
 		}
 	} else if c.CredentialSource.File != "" {