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/aws_test.go

@@ -1234,3 +1234,20 @@ func TestAWSCredential_ShouldCallMetadataEndpointWhenNoSecretAccessKey(t *testin
 		t.Errorf("subjectToken = \n%q\n want \n%q", got, want)
 	}
 }
+
+func TestAwsCredential_CredentialSourceType(t *testing.T) {
+	server := createDefaultAwsTestServer()
+	ts := httptest.NewServer(server)
+
+	tfc := testFileConfig
+	tfc.CredentialSource = server.getCredentialSource(ts.URL)
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	if got, want := base.credentialSourceType(), "aws"; got != want {
+		t.Errorf("got %v but want %v", got, want)
+	}
+}