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: c93cd478e5fade98bcf846164b9b56f89b442f6b
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_test.go

@@ -68,6 +68,9 @@ func TestRetrieveFileSubjectToken(t *testing.T) {
 				t.Errorf("got %v but want %v", out, test.want)
 			}
 
+			if got, want := base.credentialSourceType(), "file"; got != want {
+				t.Errorf("got %v but want %v", got, want)
+			}
 		})
 	}
 }