When a host header is defined override `req.Host` in the metrics ui (#13071)

* When a host header is defined override the req.Host in the metrics ui endpoint.

* add changelog

.changelog/13071.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+Fix a bug when configuring an `add_headers` directive named `Host` the header is not set for `v1/internal/ui/metrics-proxy/` endpoint.
+```

agent/ui_endpoint.go

@@ -740,8 +740,12 @@ func (s *HTTPHandlers) UIMetricsProxy(resp http.ResponseWriter, req *http.Reques
 
 	// Add any configured headers
 	for _, h := range cfg.AddHeaders {
+		if strings.ToLower(h.Name) == "host" {
+			req.Host = h.Value
+		} else {
 			req.Header.Set(h.Name, h.Value)
 		}
+	}
 
 	log.Debug("proxying request", "to", u.String())