Initial pass at prometheus monitoring.

This is a step for #29118.

Add a single metric to count successful logins.

Summary types are not supported so remove Collector.  Either
we need to support the summary type or we need to create a
multiprocess-friendly Collector.

Add config to load prometheus and set up the Collector and the
Exporter.

Fix `Gemfile` as current prometheus-client gemspec is missing the
`mmap2` dependency.

Gemfile.lock

@@ -465,6 +465,7 @@ GEM
     mimemagic (0.3.0)
     mini_portile2 (2.1.0)
     minitest (5.7.0)
+    mmap2 (2.2.6)
     mousetrap-rails (1.4.6)
     multi_json (1.12.1)
     multi_xml (0.6.0)
@@ -977,6 +978,7 @@ DEPENDENCIES
   mail_room (~> 0.9.1)
   method_source (~> 0.8)
   minitest (~> 5.7.0)
+  mmap2 (~> 2.2.6)
   mousetrap-rails (~> 1.4.6)
   mysql2 (~> 0.3.16)
   net-ssh (~> 3.0.1)

app/controllers/sessions_controller.rb

@@ -34,6 +34,7 @@ class SessionsController < Devise::SessionsController
       end
       # hide the signed-in notification
       flash[:notice] = nil
+      PromService.instance.login.increment
       log_audit_event(current_user, with: authentication_method)
       log_user_activity(current_user)
     end

app/services/prom_service.rb

@@ -0,0 +1,16 @@
+require 'prometheus/client'
+require 'singleton'
+
+class PromService
+  include Singleton
+
+  attr_reader :login
+
+  def initialize
+    @prometheus = Prometheus::Client.registry
+
+    @login = Prometheus::Client::Counter.new(:login, 'Login counter')
+    @prometheus.register(@login)
+
+  end
+end

config.ru

@@ -13,6 +13,13 @@ if defined?(Unicorn)
     # Max memory size (RSS) per worker
     use Unicorn::WorkerKiller::Oom, min, max
   end
+
+  # TODO(lyda): Needs to be set externally.
+  ENV['prometheus_multiproc_dir'] = '/tmp'
+
+  require 'prometheus/client/rack/exporter'
+
+  use Prometheus::Client::Rack::Exporter, path: '/admin/metrics'
 end
 
 require ::File.expand_path('../config/environment',  __FILE__)