TD-187: Add Prometheus (#5)

config/sys.config

@@ -99,6 +99,9 @@
     {snowflake, [
         {max_backward_clock_moving, 1000}  % 1 second
         % {machine_id, hostname_hash}
-    ]}
+    ]},
 
+    {prometheus, [
+        {collectors, [default]}
+    ]}
 ].

rebar.config

@@ -93,6 +93,14 @@
 
 {profiles, [
     {prod, [
+        {deps, [
+            %% NOTE
+            %% Because of a dependency conflict, prometheus libs are only included in production build for now
+            %% https://github.com/project-fifo/rebar3_lint/issues/42
+            %% https://github.com/valitydev/hellgate/pull/2/commits/884724c1799703cee4d1033850fe32c17f986d9e
+            {prometheus, "4.8.1"},
+            {prometheus_cowboy, "0.1.8"}
+        ]},
         %% Relx configuration
         {relx, [
             {release, {bouncer, "0.1.0"}, [
@@ -106,6 +114,8 @@
                 % logger formatter
                 {logger_logstash_formatter, load},
                 how_are_you,
+                prometheus,
+                prometheus_cowboy,
                 bouncer
             ]},
             {sys_config, "./config/sys.config"},

src/bouncer.erl

@@ -32,6 +32,10 @@ init([]) ->
     EventHandlers = genlib_app:env(?MODULE, woody_event_handlers, [woody_event_handler_default]),
     Healthcheck = enable_health_logging(genlib_app:env(?MODULE, health_check, #{})),
     {OpaClient, OpaClientSpec} = bouncer_opa_client:init(get_opa_opts()),
+    AdditionalRoutes = [
+        erl_health_handle:get_route(Healthcheck),
+        get_prometheus_route()
+    ],
     WoodySpec = woody_server:child_spec(
         ?MODULE,
         #{
@@ -44,7 +48,7 @@ init([]) ->
             handlers =>
                 get_handler_specs(ServiceOpts, AuditPulse, OpaClient) ++
                 get_stub_handler_specs(ServiceOpts),
-            additional_routes => [erl_health_handle:get_route(Healthcheck)]
+            additional_routes => AdditionalRoutes
         }
     ),
     {ok, {
@@ -126,3 +130,7 @@ enable_health_logging(Check) ->
         fun(_, Runner) -> #{runner => Runner, event_handler => EvHandler} end,
         Check
     ).
+
+-spec get_prometheus_route() -> {iodata(), module(), _Opts :: any()}.
+get_prometheus_route() ->
+    {"/metrics/[:registry]", prometheus_cowboy2_handler, []}.