Pass HTTP transport to the proxy

When we proxy requests we should use same transport as we use when collecting alerts & silences

internal/alertmanager/models.go

@@ -40,7 +40,7 @@ type Alertmanager struct {
 	reader uri.Reader
 	// implements how we fetch requests from the Alertmanager, we don't set it
 	// by default so it's nil and http.DefaultTransport is used
-	httpTransport http.RoundTripper
+	HTTPTransport http.RoundTripper `json:"-"`
 	// lock protects data access while updating
 	lock sync.RWMutex
 	// fields for storing pulled data

internal/alertmanager/upstream.go

@@ -46,7 +46,7 @@ func NewAlertmanager(name, upstreamURI string, opts ...Option) (*Alertmanager, e
 	}
 
 	var err error
-	am.reader, err = uri.NewReader(am.URI, am.RequestTimeout, am.httpTransport)
+	am.reader, err = uri.NewReader(am.URI, am.RequestTimeout, am.HTTPTransport)
 	if err != nil {
 		return am, err
 	}
@@ -112,7 +112,7 @@ func WithRequestTimeout(timeout time.Duration) Option {
 // a custom HTTP transport (http.RoundTripper implementation)
 func WithHTTPTransport(httpTransport http.RoundTripper) Option {
 	return func(am *Alertmanager) error {
-		am.httpTransport = httpTransport
+		am.HTTPTransport = httpTransport
 		return nil
 	}
 }

proxy.go

@@ -37,6 +37,7 @@ func NewAlertmanagerProxy(alertmanager *alertmanager.Alertmanager) (*httputil.Re
 			req.Header.Del("Accept-Encoding")
 			log.Debugf("[%s] Proxy request for %s", alertmanager.Name, req.URL.Path)
 		},
+		Transport: alertmanager.HTTPTransport,
 		ModifyResponse: func(resp *http.Response) error {
 			// drop Content-Length header from upstream responses, gzip middleware
 			// will compress those and that could cause a mismatch