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/basecredentials.go

@@ -198,6 +198,7 @@ func (c *Config) parse(ctx context.Context) (baseCredentialSource, error) {
 }
 
 type baseCredentialSource interface {
+	credentialSourceType() string
 	subjectToken() (string, error)
 }
 
@@ -207,6 +208,15 @@ type tokenSource struct {
 	conf *Config
 }
 
+func getMetricsHeaderValue(conf *Config, credSource baseCredentialSource) string {
+	return fmt.Sprintf("gl-go/%s auth/%s google-byoid-sdk source/%s sa-impersonation/%t config-lifetime/%t",
+		goVersion(),
+		"unknown",
+		credSource.credentialSourceType(),
+		conf.ServiceAccountImpersonationURL != "",
+		conf.ServiceAccountImpersonationLifetimeSeconds != 0)
+}
+
 // Token allows tokenSource to conform to the oauth2.TokenSource interface.
 func (ts tokenSource) Token() (*oauth2.Token, error) {
 	conf := ts.conf
@@ -230,6 +240,7 @@ func (ts tokenSource) Token() (*oauth2.Token, error) {
 	}
 	header := make(http.Header)
 	header.Add("Content-Type", "application/x-www-form-urlencoded")
+	header.Add("x-goog-api-client", getMetricsHeaderValue(conf, credSource))
 	clientAuth := clientAuthentication{
 		AuthStyle:    oauth2.AuthStyleInHeader,
 		ClientID:     conf.ClientID,