Expose Prometheus metrics at /metrics

Exposes metrics:

* http_request_duration_seconds_bucket
* http_request_size_bytes_bucket
* http_response_size_bytes_bucket
* http_requests_total

with labels of action and controller to help identify popular
endpoints and their performance characteristics.

flake.nix

@@ -70,6 +70,21 @@
             };
           };
 
+          CatalystPluginPrometheusTiny = final.buildPerlPackage {
+            pname = "Catalyst-Plugin-PrometheusTiny";
+            version = "0.005";
+            src = final.fetchurl {
+              url = "mirror://cpan/authors/id/S/SY/SYSPETE/Catalyst-Plugin-PrometheusTiny-0.005.tar.gz";
+              sha256 = "a42ef09efdc3053899ae007c41220d3ed7207582cc86e491b4f534539c992c5a";
+            };
+            buildInputs = with final.perlPackages; [ HTTPMessage Plack SubOverride TestDeep ];
+            propagatedBuildInputs = with final.perlPackages; [ CatalystRuntime Moose PrometheusTiny PrometheusTinyShared ];
+            meta = {
+              description = "Prometheus metrics for Catalyst";
+              license = with final.lib.licenses; [ artistic1 gpl1Plus ];
+            };
+          };
+
           CryptArgon2 = final.perlPackages.buildPerlModule {
             pname = "Crypt-Argon2";
             version = "0.010";
@@ -358,6 +373,7 @@
                 CatalystPluginAccessLog
                 CatalystPluginAuthorizationRoles
                 CatalystPluginCaptcha
+                CatalystPluginPrometheusTiny
                 CatalystPluginSessionStateCookie
                 CatalystPluginSessionStoreFastMmap
                 CatalystPluginStackTrace

src/lib/Hydra.pm

@@ -16,7 +16,8 @@ use Catalyst qw/ConfigLoader
                 Session
                 Session::Store::FastMmap
                 Session::State::Cookie
-                Captcha/,
+                Captcha
+                PrometheusTiny/,
                 '-Log=warn,fatal,error';
 use CatalystX::RoleApplicator;
 use YAML qw(LoadFile);
@@ -47,6 +48,9 @@ __PACKAGE__->config(
             file($ENV{'HYDRA_LDAP_CONFIG'})
         ) : undef
     },
+    'Plugin::PrometheusTiny' => {
+        include_action_labels => 1,
+    },
     'Plugin::Static::Simple' => {
         send_etag => 1,
         expires => 3600

t/Controller/metrics.t

@@ -0,0 +1,30 @@
+use feature 'unicode_strings';
+use strict;
+use Setup;
+use JSON qw(decode_json encode_json);
+
+my %ctx = test_init();
+
+require Hydra::Schema;
+require Hydra::Model::DB;
+require Hydra::Helper::Nix;
+use HTTP::Request::Common;
+
+use Test2::V0;
+require Catalyst::Test;
+Catalyst::Test->import('Hydra');
+
+my $db = Hydra::Model::DB->new;
+hydra_setup($db);
+
+request(GET '/');
+my $metrics = request(GET '/metrics');
+ok($metrics->is_success);
+
+like(
+    $metrics->content,
+    qr/http_requests_total\{action="index",code="200",controller="Hydra::Controller::Root",method="GET"\} 1/,
+    "Metrics are collected"
+);
+
+done_testing;