Add instrumentation callback for modules

It returns a list of instrument specs, just like hooks/1 returns hooks.

Instrumentation is set up before hooks, and torn down after them.

src/gen_mod.erl

@@ -72,6 +72,7 @@
 -callback hooks(HostType :: host_type()) -> gen_hook:hook_list().
 -callback supported_features() -> [module_feature()].
 -callback config_spec() -> mongoose_config_spec:config_section().
+-callback instrumentation(host_type()) -> [mongoose_instrument:spec()].
 
 %% Optional callback specifying module dependencies.
 %% The dependent module can specify parameters with which the dependee should be
@@ -86,7 +87,7 @@
 %% function).
 -callback deps(host_type(), module_opts()) -> gen_mod_deps:deps().
 
--optional_callbacks([hooks/1, config_spec/0, supported_features/0, deps/2]).
+-optional_callbacks([hooks/1, config_spec/0, supported_features/0, instrumentation/1, deps/2]).
 
 %% @doc This function should be called by mongoose_modules only.
 %% To start a new module at runtime, use mongoose_modules:ensure_module/3 instead.
@@ -101,6 +102,7 @@ start_module_for_host_type(HostType, Module, Opts) ->
         lists:map(fun mongoose_service:assert_loaded/1,
                   get_required_services(HostType, Module, Opts)),
         check_dynamic_domains_support(HostType, Module),
+        run_for_instrumentation(HostType, fun mongoose_instrument:set_up/1, Module),
         Res = Module:start(HostType, Opts),
         run_for_hooks(HostType, fun gen_hook:add_handlers/1, Module),
         {links, LinksAfter} = erlang:process_info(self(), links),
@@ -152,6 +154,12 @@ run_for_hooks(HostType, Fun, Module) ->
         false -> ok
     end.
 
+run_for_instrumentation(HostType, Fun, Module) ->
+    case erlang:function_exported(Module, instrumentation, 1) of
+        true -> Fun(Module:instrumentation(HostType));
+        false -> ok
+    end.
+
 check_dynamic_domains_support(HostType, Module) ->
     case lists:member(HostType, ?MYHOSTS) of
         true -> ok;
@@ -197,7 +205,7 @@ stop_module_for_host_type(HostType, Module) ->
         {wait, Process} ->
             wait_for_process(Process);
         _ ->
-            ok
+            run_for_instrumentation(HostType, fun mongoose_instrument:tear_down/1, Module)
     catch Class:Reason:Stacktrace ->
             ?LOG_ERROR(#{what => module_stopping_failed,
                          host_type => HostType, stop_module => Module,