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/executablecredsource_test.go

@@ -150,6 +150,9 @@ func TestCreateExecutableCredential(t *testing.T) {
 				if ecs.Timeout != tt.expectedTimeout {
 					t.Errorf("ecs.Timeout got %v but want %v", ecs.Timeout, tt.expectedTimeout)
 				}
+				if ecs.credentialSourceType() != "executable" {
+					t.Errorf("ecs.CredentialSourceType() got %s but want executable", ecs.credentialSourceType())
+				}
 			}
 		})
 	}