feat: add health server and prometheus metrics to worker

app/util/has_prometheus_metrics.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module HasPrometheusMetrics
+
+  def register_prometheus_counter(name, **kwargs)
+    counter = Prometheus::Client::Counter.new(name, **kwargs)
+    registry.register(counter)
+  end
+
+  def increment_prometheus_counter(name, labels: {})
+    counter = registry.get(name)
+    return if counter.nil?
+
+    counter.increment(labels: labels)
+  end
+
+  private
+
+  def registry
+    Prometheus::Client.registry
+  end
+
+end

app/util/health_server.rb

@@ -0,0 +1,107 @@
+# frozen_string_literal: true
+
+require "socket"
+require "rack/handler/webrick"
+require "prometheus/client/formats/text"
+
+class HealthServer
+
+  def initialize(name: "unnamed-process")
+    @name = name
+  end
+
+  def call(env)
+    case env["PATH_INFO"]
+    when "/health"
+      ok
+    when "/metrics"
+      metrics
+    when "/"
+      root
+    else
+      not_found
+    end
+  end
+
+  private
+
+  def root
+    [200, { "Content-Type" => "text/plain" }, ["#{@name} (pid: #{Process.pid}, host: #{Socket.gethostname})"]]
+  end
+
+  def ok
+    [200, { "Content-Type" => "text/plain" }, ["OK"]]
+  end
+
+  def not_found
+    [404, { "Content-Type" => "text/plain" }, ["Not Found"]]
+  end
+
+  def metrics
+    registry = Prometheus::Client.registry
+    body = Prometheus::Client::Formats::Text.marshal(registry)
+    [200, { "Content-Type" => "text/plain" }, [body]]
+  end
+
+  class << self
+
+    def run(default_port: 9090, **options)
+      port = ENV.fetch("HEALTH_SERVER_PORT", default_port)
+
+      Rack::Handler::WEBrick.run(new(**options),
+                                 Port: port,
+                                 BindAddress: bind_address,
+                                 AccessLog: [],
+                                 Logger: LoggerProxy.new)
+    rescue Errno::EADDRINUSE
+      Postal.logger.info "health server port (#{bind_address}:#{port}) is already " \
+                         "in use, not starting health server"
+    end
+
+    def bind_address
+      ENV.fetch("HEALTH_SERVER_BIND_ADDRESS", "127.0.0.1")
+    end
+
+    def start(**options)
+      thread = Thread.new { run(**options) }
+      thread.abort_on_exception = false
+      thread
+    end
+
+  end
+
+  class LoggerProxy
+
+    [:info, :debug, :warn, :error, :fatal].each do |severity|
+      define_method(severity) do |message|
+        add(severity, message)
+      end
+
+      define_method("#{severity}?") do
+        severity != :debug
+      end
+    end
+
+    def add(severity, message)
+      return if severity == :debug
+
+      case message
+      when /\AWEBrick::HTTPServer#start:.*port=(\d+)/
+        Postal.logger.info "started health server on port #{::Regexp.last_match(1)}", component: "true"
+      when /\AWEBrick::HTTPServer#start done/
+        Postal.logger.info "stopped health server"
+      when /\AWEBrick [\d.]+/,
+           /\Aruby ([\d.]+)/,
+           /\ARack::Handler::WEBrick is mounted/,
+           /\Aclose TCPSocket/,
+           /\Agoing to shutdown/
+        # Don't actually print routine messages to avoid too much
+        # clutter when processes start it
+      else
+        Postal.logger.debug message, component: "true"
+      end
+    end
+
+  end
+
+end