Add the ability to inject a custom round tripper

config/http_config.go

@@ -80,6 +80,9 @@ func (u URL) MarshalYAML() (interface{}, error) {
 	return nil, nil
 }
 
+// RoundTripWrapper is used in HTTPClientConfig to add custom functionality to the http request path
+type RoundTripWrapper func(rt http.RoundTripper) http.RoundTripper
+
 // HTTPClientConfig configures an HTTP client.
 type HTTPClientConfig struct {
 	// The HTTP basic authentication credentials for the targets.
@@ -92,6 +95,8 @@ type HTTPClientConfig struct {
 	ProxyURL URL `yaml:"proxy_url,omitempty"`
 	// TLSConfig to use to connect to the targets.
 	TLSConfig TLSConfig `yaml:"tls_config,omitempty"`
+	// WrapBaseRoundTripper can be used to add custom functionality in the http request path
+	WrapBaseRoundTripper RoundTripWrapper `yaml:"-"`
 }
 
 // SetDirectory joins any relative file paths with dir.
@@ -186,6 +191,10 @@ func NewRoundTripperFromConfig(cfg HTTPClientConfig, name string, disableKeepAli
 			}
 		}
 
+		if cfg.WrapBaseRoundTripper != nil {
+			rt = cfg.WrapBaseRoundTripper(rt)
+		}
+
 		// If a bearer token is provided, create a round tripper that will set the
 		// Authorization header correctly on each request.
 		if len(cfg.BearerToken) > 0 {

config/http_config_test.go

@@ -56,6 +56,8 @@ const (
 	ExpectedBearer         = "Bearer " + BearerToken
 	ExpectedUsername       = "arthurdent"
 	ExpectedPassword       = "42"
+	ExpectedHeader         = "slartibartfast"
+	ExpectedHeaderValue    = "fjords"
 )
 
 var invalidHTTPClientConfigs = []struct {
@@ -76,6 +78,13 @@ var invalidHTTPClientConfigs = []struct {
 	},
 }
 
+type roundTripperFunc func(req *http.Request) (*http.Response, error)
+
+// RoundTrip implements the RoundTripper interface.
+func (rt roundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) {
+	return rt(r)
+}
+
 func newTestServer(handler func(w http.ResponseWriter, r *http.Request)) (*httptest.Server, error) {
 	testServer := httptest.NewUnstartedServer(http.HandlerFunc(handler))
 
@@ -104,6 +113,14 @@ func newTestServer(handler func(w http.ResponseWriter, r *http.Request)) (*httpt
 }
 
 func TestNewClientFromConfig(t *testing.T) {
+	wrapper := func(rt http.RoundTripper) http.RoundTripper {
+		return roundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			req.Header.Add(ExpectedHeader, ExpectedHeaderValue)
+
+			return rt.RoundTrip(req)
+		})
+	}
+
 	var newClientValidConfig = []struct {
 		clientConfig HTTPClientConfig
 		handler      func(w http.ResponseWriter, r *http.Request)
@@ -195,6 +212,25 @@ func TestNewClientFromConfig(t *testing.T) {
 					fmt.Fprint(w, ExpectedMessage)
 				}
 			},
+		}, {
+			clientConfig: HTTPClientConfig{
+				TLSConfig: TLSConfig{
+					CAFile:             "",
+					CertFile:           ClientCertificatePath,
+					KeyFile:            ClientKeyNoPassPath,
+					ServerName:         "",
+					InsecureSkipVerify: true},
+				WrapBaseRoundTripper: wrapper,
+			},
+			handler: func(w http.ResponseWriter, r *http.Request) {
+				val := r.Header.Get(ExpectedHeader)
+				if val != ExpectedHeaderValue {
+					fmt.Fprintf(w, "The expected Header Value (%s) differs from the obtained Header Value (%s)",
+						ExpectedHeaderValue, val)
+				} else {
+					fmt.Fprint(w, ExpectedMessage)
+				}
+			},
 		},
 	}