Set Host field appropriately when passing through probes (#20371)

* Set Host field appropriately when passing through probes

If someone sets a Host header, we shouldn't overwrite it back to
localhost. The health check might be affected by the value of that
header. We should ensure the host field is set to the provided host
header, if set on the probe

* cover :authority too

pilot/cmd/pilot-agent/status/server.go

@@ -244,6 +244,14 @@ func (s *Server) handleAppProbe(w http.ResponseWriter, req *http.Request) {
 		appReq.Header[name] = newValues
 	}
 
+	for _, h := range prober.HTTPHeaders {
+		if h.Name == "Host" || h.Name == ":authority" {
+			// Probe has specific host header override; honor it
+			appReq.Host = h.Value
+			break
+		}
+	}
+
 	// Send the request.
 	response, err := httpClient.Do(appReq)
 	if err != nil {