Adds the ability to provide a tripperware to Promtail client.

clients/pkg/promtail/client/client.go

@@ -157,8 +157,15 @@ type client struct {
 	cancel context.CancelFunc
 }
 
+// Tripperware can wrap a roundtripper.
+type Tripperware func(http.RoundTripper) http.RoundTripper
+
 // New makes a new Client.
 func New(reg prometheus.Registerer, cfg Config, logger log.Logger) (Client, error) {
+	return newClient(reg, cfg, logger)
+}
+
+func newClient(reg prometheus.Registerer, cfg Config, logger log.Logger) (*client, error) {
 	if cfg.URL.URL == nil {
 		return nil, errors.New("client needs target URL")
 	}
@@ -199,6 +206,20 @@ func New(reg prometheus.Registerer, cfg Config, logger log.Logger) (Client, erro
 	return c, nil
 }
 
+// NewWithTripperware creates a new Loki client with a custom tripperware.
+func NewWithTripperware(reg prometheus.Registerer, cfg Config, logger log.Logger, tp Tripperware) (Client, error) {
+	c, err := newClient(reg, cfg, logger)
+	if err != nil {
+		return nil, err
+	}
+
+	if tp != nil {
+		c.client.Transport = tp(c.client.Transport)
+	}
+
+	return c, nil
+}
+
 func (c *client) run() {
 	batches := map[string]*batch{}
 

clients/pkg/promtail/client/client_test.go

@@ -1,9 +1,11 @@
 package client
 
 import (
+	"io"
 	"math"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"strings"
 	"testing"
 	"time"
@@ -459,3 +461,35 @@ func createServerHandler(receivedReqsChan chan receivedReq, status int) http.Han
 		rw.WriteHeader(status)
 	})
 }
+
+type RoundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (r RoundTripperFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return r(req)
+}
+
+func Test_Tripperware(t *testing.T) {
+	url, err := url.Parse("http://foo.com")
+	require.NoError(t, err)
+	var called bool
+	c, err := NewWithTripperware(nil, Config{
+		URL: flagext.URLValue{URL: url},
+	}, log.NewNopLogger(), func(rt http.RoundTripper) http.RoundTripper {
+		return RoundTripperFunc(func(r *http.Request) (*http.Response, error) {
+			require.Equal(t, r.URL.String(), "http://foo.com")
+			called = true
+			return &http.Response{
+				StatusCode: http.StatusOK,
+				Body:       io.NopCloser(strings.NewReader("ok")),
+			}, nil
+		})
+	})
+	require.NoError(t, err)
+
+	c.Chan() <- api.Entry{
+		Labels: model.LabelSet{"foo": "bar"},
+		Entry:  logproto.Entry{Timestamp: time.Now(), Line: "foo"},
+	}
+	c.Stop()
+	require.True(t, called)
+}