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.

2 files changed, 17 insertions, 0 deletions

diff --git a/app/controllers/sessions_controller.rb b/app/controllers/sessions_controller.rb
index 8c6ba4915cd..9870d4286a6 100644
--- a/app/controllers/sessions_controller.rb
+++ b/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
diff --git a/app/services/prom_service.rb b/app/services/prom_service.rb
new file mode 100644
index 00000000000..e9ccce758e9
--- /dev/null
+++ b/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