Some fixes suggested by golangci-lint

internal/server/model/handler.go

@@ -5,10 +5,25 @@ import (
 	"sync"
 )
 
+// Handler represents an open HTTP connection in the User Server.
+//
+// This struct contains the connection Writer and Request to be managed
+// by endpoints of the Data Server.
 type Handler struct {
-	Id      string
-	Route   *Route
+	// ID is unique identifier of the request.
+	ID string
+
+	// Route is a reference to the original route that matched this
+	// request.
+	Route *Route
+
+	// Writing is a mutex that prevents two goroutines from writing at
+	// the same time in the response.
 	Writing sync.Mutex
-	Writer  http.ResponseWriter
+
+	// Request is a pointer to the in-progress request.
 	Request *http.Request
+
+	// Writer is the original http.ResponseWriter of the request.
+	Writer http.ResponseWriter
 }