refactor(emqx_prometheus_schema): use typerefl alias

apps/emqx_prometheus/src/emqx_prometheus_schema.erl

@@ -57,7 +57,7 @@ fields("prometheus") ->
             )},
         {headers,
             ?HOCON(
-                list({string(), string()}),
+                typerefl:alias("map", list({string(), string()})),
                 #{
                     default => #{},
                     required => false,