google/internal/externalaccount: adding BYOID Metrics

Adds framework for sending BYOID metrics via the x-goog-api-client header on outgoing sts requests. Also adds a header file for getting the current version of GoLang

Change-Id: Id5431def96f4cfc03e4ada01d5fb8cac8cfa56a9
GitHub-Last-Rev: c93cd478e5
GitHub-Pull-Request: golang/oauth2#661
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/523595
Reviewed-by: Leo Siracusa <leosiracusa@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>

google/internal/externalaccount/filecredsource.go

@@ -19,6 +19,10 @@ type fileCredentialSource struct {
 	Format format
 }
 
+func (cs fileCredentialSource) credentialSourceType() string {
+	return "file"
+}
+
 func (cs fileCredentialSource) subjectToken() (string, error) {
 	tokenFile, err := os.Open(cs.File)
 	if err != nil {