Use the gem for the ruby prometheus client.

Gemfile.lock

-GIT
-  remote: https://github.com/prometheus/client_ruby.git
-  revision: 30dc5987c6bf574044d75d97362bd3bd27d542f2
-  branch: exchangeable-value-types
-  specs:
-    prometheus-client (0.7.0.beta1)
-      quantile (~> 0.2.0)
-
 GEM
   remote: https://rubygems.org/
   specs:
@@ -569,6 +561,8 @@ GEM
     premailer-rails (1.9.2)
       actionmailer (>= 3, < 6)
       premailer (~> 1.7, >= 1.7.9)
+    prometheus-client-mmap (0.7.0.beta1)
+      quantile (~> 0.2.0)
     pry (0.10.4)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
@@ -1006,7 +1000,7 @@ DEPENDENCIES
   pg (~> 0.18.2)
   poltergeist (~> 1.9.0)
   premailer-rails (~> 1.9.0)
-  prometheus-client!
+  prometheus-client-mmap (~> 0.7.0.beta1)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
   rack-attack (~> 4.4.1)