Add session_lifetime metric

src/mtp_handler.erl

@@ -168,8 +168,12 @@ handle_info(Other, S) ->
     lager:warning("Unexpected handle_info ~p", [Other]),
     {noreply, S}.
 
-terminate(_Reason, #state{}) ->
+terminate(_Reason, #state{started_at = Started}) ->
     mtp_metric:count_inc([?APP, in_connection_closed, total], 1, #{}),
+    Lifetime = erlang:system_time(millisecond) - Started,
+    metric:histogram_observe(
+      [?APP, session_lifetime, seconds],
+      erlang:convert_time_unit(Lifetime, millisecond, native), #{}),
     lager:debug("terminate ~p", [_Reason]),
     ok.