feat: support Prometheus exports

We package a quite old plugin for Buildbot: https://github.com/claws/buildbot-prometheus
Ideally, we should probably vendor it and maintain it ourselves.

There seems to be no protection against the metrics endpoint for
Buildbot, this is not a big deal given that the CI is public.

Signed-off-by: Raito Bezarius <raito@lix.systems>

buildbot_nix/__init__.py

@@ -795,6 +795,7 @@ class GerritNixConfigurator(ConfiguratorBase):
         nix_eval_max_memory_size: int,
         nix_workers_secret_name: str = "buildbot-nix-workers",  # noqa: S107
         signing_keyfile: str | None = None,
+        prometheus_config: dict[str, int | str] | None = None,
         binary_cache_config: dict[str, str] | None = None,
         outputs_path: str | None = None,
     ) -> None:
@@ -808,6 +809,7 @@ class GerritNixConfigurator(ConfiguratorBase):
         self.nix_supported_systems = nix_supported_systems
         self.gerrit_change_source = GerritChangeSource(gerrit_server, gerrit_user, gerritport=gerrit_port, identity_file=gerrit_sshkey_path)
         self.url = url
+        self.prometheus_config = prometheus_config
         if binary_cache_config is not None:
             self.binary_cache_config = S3BinaryCacheConfig(**binary_cache_config)
         else:
@@ -867,6 +869,9 @@ class GerritNixConfigurator(ConfiguratorBase):
 
         )
 
+        if self.prometheus_config is not None:
+            config['services'].append(reporters.Prometheus(port=self.prometheus_config.get('port', 9100), interface=self.prometheus_config.get('address', '')))
+
         def gerritBranchKey(b):
             ref = b['branch']
             if not ref.startswith('refs/changes/'):

nix/coordinator.nix

@@ -65,6 +65,20 @@ in
         example = "/run/agenix.d/signing-key";
       };
 
+      prometheus = {
+        enable = lib.mkEnableOption " the export of metrics in Prometheus format";
+        address = lib.mkOption {
+          type = lib.types.str;
+          default = "";
+          description = "The local IPv4 or IPv6 address to which to bind; defaults to '' represents all IPv4 addresses.";
+        };
+        port = lib.mkOption {
+          type = lib.types.port;
+          default = 9100;
+          description = "A port on which the metrics endpoint will be available";
+        };
+      };
+
       binaryCache = {
         enable = lib.mkEnableOption " binary cache upload to a S3 bucket";
         profileCredentialsFile = lib.mkOption {
@@ -132,6 +146,9 @@ in
               nix_eval_worker_count=${if cfg.evalWorkerCount == null then "None" else builtins.toString cfg.evalWorkerCount},
               nix_supported_systems=${builtins.toJSON cfg.buildSystems},
               outputs_path=${if cfg.outputsPath == null then "None" else builtins.toJSON cfg.outputsPath},
+              prometheus_config=${if (!cfg.prometheus.enable) then "None" else builtins.toJSON {
+                inherit (cfg.prometheus) address port;
+              }}
               # Signing key file must be available on the workers and readable.
               signing_keyfile=${if cfg.signingKeyFile == null then "None" else builtins.toJSON cfg.signingKeyFile},
               binary_cache_config=${if (!cfg.binaryCache.enable) then "None" else builtins.toJSON {
@@ -148,14 +165,15 @@ in
         in
         "${if hasSSL then "https" else "http"}://${cfg.domain}/";
       dbUrl = cfg.dbUrl;
-      pythonPackages = ps: [
+      pythonPackages = ps: ([
         ps.requests
         ps.treq
         ps.psycopg2
         (ps.toPythonModule pkgs.buildbot-worker)
         pkgs.buildbot-plugins.www
         (pkgs.python3.pkgs.callPackage ../default.nix { })
-      ];
+      ] ++ lib.optional cfg.prometheus.enable (pkgs.python3.pkgs.callPackage ./prometheus-plugin.nix { })
+      );
     };
 
     # TODO(raito): we assume worker runs on coordinator. please clean up this later.

nix/prometheus-plugin.nix

@@ -0,0 +1,42 @@
+{ lib
+, buildPythonPackage
+, fetchFromGitHub
+, setuptools
+, wheel
+, buildbot
+, prometheus-client
+, twisted
+}:
+
+buildPythonPackage rec {
+  pname = "buildbot-prometheus";
+  version = "unstable-2024-05-06";
+  pyproject = true;
+
+  src = fetchFromGitHub {
+    owner = "claws";
+    repo = "buildbot-prometheus";
+    rev = "0c81a89bbe34628362652fbea416610e215b5d1e";
+    hash = "sha256-bz2Nv2RZ44i1VoPvQ/XjGMfTT6TmW6jhEVwItPk23SM=";
+  };
+
+  nativeBuildInputs = [
+    setuptools
+    wheel
+  ];
+
+  propagatedBuildInputs = [
+    buildbot
+    prometheus-client
+    twisted
+  ];
+
+  pythonImportsCheck = [ "buildbot_prometheus" ];
+
+  meta = with lib; {
+    description = "";
+    homepage = "https://github.com/claws/buildbot-prometheus";
+    license = licenses.mit;
+    maintainers = with maintainers; [ raitobezarius ];
+  };
+}