Middleware: Add Custom Headers to HTTP responses (#59018)

* Middleware: Add Custom Headers to HTTP responses

* Update docs/sources/setup-grafana/configure-grafana/_index.md

Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

* Update conf/defaults.ini

Co-authored-by: Dave Henderson <dave.henderson@grafana.com>

* Update conf/sample.ini

Co-authored-by: Dave Henderson <dave.henderson@grafana.com>

* Update _index.md

Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>
Co-authored-by: Dave Henderson <dave.henderson@grafana.com>

pkg/setting/setting.go

@@ -486,6 +486,8 @@ type Cfg struct {
 	GRPCServerNetwork   string
 	GRPCServerAddress   string
 	GRPCServerTLSConfig *tls.Config
+
+	CustomResponseHeaders map[string]string
 }
 
 type CommandLineArgs struct {
@@ -1692,6 +1694,14 @@ func (cfg *Cfg) readServerSettings(iniFile *ini.File) error {
 
 	cfg.ReadTimeout = server.Key("read_timeout").MustDuration(0)
 
+	headersSection := cfg.Raw.Section("server.custom_response_headers")
+	keys := headersSection.Keys()
+	cfg.CustomResponseHeaders = make(map[string]string, len(keys))
+
+	for _, key := range keys {
+		cfg.CustomResponseHeaders[key.Name()] = key.Value()
+	}
+
 	return nil
 }