Start local prometheus_exporter server only in puma/sidekiq startup (#35005)

lib/mastodon/prometheus_exporter/local_server.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'prometheus_exporter/server'
+require 'prometheus_exporter/client'
+
+module Mastodon::PrometheusExporter
+  module LocalServer
+    mattr_accessor :bind, :port
+
+    def self.setup!
+      server = PrometheusExporter::Server::WebServer.new(bind:, port:)
+      server.start
+
+      # wire up a default local client
+      PrometheusExporter::Client.default = PrometheusExporter::LocalClient.new(collector: server.collector)
+    end
+  end
+end