documentation: add tail_proxy_url option to query_frontend_config section (#2879)

* add tail_proxy_url option to the query_frontend_config section of configuration doc

* If queryFrontend run in tail proxy mode, append the Host of the tail_proxy_url to the request header, because in some scenario queryFrontend forward tail to the queriers deployed behind nginx and "Host" is required.

docs/sources/configuration/_index.md

@@ -296,6 +296,10 @@ The query_frontend_config configures the Loki query-frontend.
 # Set to < 0 to enable on all queries.
 # CLI flag: -frontend.log-queries-longer-than
 [log_queries_longer_than: <duration> | default = 0s]
+
+# URL of querier for tail proxy.
+# CLI flag: -frontend.tail-proxy-url
+[tail_proxy_url: <string> | default = ""]
 ```
 
 ## queryrange_config

pkg/loki/modules.go

@@ -383,6 +383,13 @@ func (t *Loki) initQueryFrontend() (_ services.Service, err error) {
 			return nil, err
 		}
 		tp := httputil.NewSingleHostReverseProxy(tailURL)
+
+		director := tp.Director
+		tp.Director = func(req *http.Request) {
+			director(req)
+			req.Host = tailURL.Host
+		}
+
 		defaultHandler = httpMiddleware.Wrap(tp)
 	} else {
 		defaultHandler = frontendHandler