http_proxy: add proxy errors metric

Fixes #458

http_error.go

@@ -39,30 +39,33 @@ func (hp *HTTPProxy) errorResponse(req *http.Request, err error) *http.Response
 	}
 
 	var (
-		code int
-		msg  string
+		code       int
+		msg, label string
 	)
 	for _, h := range handlers {
-		code, msg = h(req, err)
+		code, msg, label = h(req, err)
 		if code != 0 {
 			break
 		}
 	}
 	if code == 0 {
 		code = http.StatusInternalServerError
 		msg = "An unexpected error occurred"
+		label = "unexpected_error"
 	}
 
+	hp.metrics.error(label)
+
 	resp := proxyutil.NewResponse(code, bytes.NewBufferString(msg+"\n"), req)
 	resp.Header.Set(ErrorHeader, err.Error())
 	resp.Header.Set("Content-Type", "text/plain; charset=utf-8")
 	resp.ContentLength = int64(len(msg) + 1)
 	return resp
 }
 
-type errorHandler func(*http.Request, error) (int, string)
+type errorHandler func(*http.Request, error) (int, string, string)
 
-func handleNetError(_ *http.Request, err error) (code int, msg string) {
+func handleNetError(_ *http.Request, err error) (code int, msg, label string) {
 	var netErr *net.OpError
 	if errors.As(err, &netErr) {
 		if netErr.Timeout() {
@@ -72,36 +75,40 @@ func handleNetError(_ *http.Request, err error) (code int, msg string) {
 			code = http.StatusBadGateway
 			msg = "Failed to connect to remote host"
 		}
+		label = "net_" + netErr.Op
 	}
 
 	return
 }
 
-func handleTLSRecordHeader(_ *http.Request, err error) (code int, msg string) {
+func handleTLSRecordHeader(_ *http.Request, err error) (code int, msg, label string) {
 	var headerErr *tls.RecordHeaderError
 	if errors.As(err, &headerErr) {
 		code = http.StatusBadGateway
 		msg = "TLS handshake failed"
+		label = "tls_record_header"
 	}
 
 	return
 }
 
-func handleTLSCertificateError(_ *http.Request, err error) (code int, msg string) {
+func handleTLSCertificateError(_ *http.Request, err error) (code int, msg, label string) {
 	var certErr *tls.CertificateVerificationError
 	if errors.As(err, &certErr) {
 		code = http.StatusBadGateway
 		msg = "TLS handshake failed"
+		label = "tls_certificate"
 	}
 
 	return
 }
 
-func handleDenyError(req *http.Request, err error) (code int, msg string) {
+func handleDenyError(req *http.Request, err error) (code int, msg, label string) {
 	var denyErr denyError
 	if errors.As(err, &denyErr) {
 		code = http.StatusForbidden
 		msg = fmt.Sprintf("proxying is denied to host %q", req.Host)
+		label = "denied"
 	}
 
 	return
@@ -110,11 +117,11 @@ func handleDenyError(req *http.Request, err error) (code int, msg string) {
 // There is a difference between sending HTTP and HTTPS requests in the presence of an upstream proxy.
 // For HTTPS client issues a CONNECT request to the proxy and then sends the original request.
 // In case the proxy responds with status code 4XX or 5XX to the CONNECT request, the client interprets it as URL error.
-func handleStatusText(req *http.Request, err error) (code int, msg string) {
+func handleStatusText(req *http.Request, err error) (code int, msg, label string) {
 	if req.URL.Scheme == "https" && err != nil {
 		for i := 400; i < 600; i++ {
 			if err.Error() == http.StatusText(i) {
-				return i, err.Error()
+				return i, err.Error(), "https_status_text"
 			}
 		}
 	}

http_proxy.go

@@ -134,6 +134,7 @@ type HTTPProxy struct {
 	creds      *CredentialsMatcher
 	transport  http.RoundTripper
 	log        log.Logger
+	metrics    *httpProxyMetrics
 	proxy      *martian.Proxy
 	mitmCACert *x509.Certificate
 	proxyFunc  ProxyFunc
@@ -200,6 +201,7 @@ func newHTTPProxy(cfg *HTTPProxyConfig, pr PACResolver, cm *CredentialsMatcher,
 		creds:     cm,
 		transport: rt,
 		log:       log,
+		metrics:   newMetrics(cfg.PromRegistry, cfg.PromNamespace),
 	}
 
 	if err := hp.configureProxy(); err != nil {

http_proxy_metrics.go

@@ -0,0 +1,35 @@
+// Copyright 2023 Sauce Labs Inc. All rights reserved.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package forwarder
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+type httpProxyMetrics struct {
+	errors *prometheus.CounterVec
+}
+
+func newMetrics(r prometheus.Registerer, namespace string) *httpProxyMetrics {
+	if r == nil {
+		r = prometheus.NewRegistry() // This registry will be discarded.
+	}
+	f := promauto.With(r)
+
+	return &httpProxyMetrics{
+		errors: f.NewCounterVec(prometheus.CounterOpts{
+			Name:      "proxy_errors_total",
+			Namespace: namespace,
+			Help:      "Number of proxy errors",
+		}, []string{"reason"}),
+	}
+}
+
+func (m *httpProxyMetrics) error(reason string) {
+	m.errors.WithLabelValues(reason).Inc()
+}