Merge branch 'ab-server-config-defaults' into 'master'

Fix error on missing `server` config section

See merge request gitlab-org/gitlab-monitor!87

Gemfile.lock

 PATH
   remote: .
   specs:
-    gitlab-monitor (3.3.0)
+    gitlab-monitor (3.3.1)
       connection_pool (~> 2.2.1)
       pg (~> 1.1)
       quantile (~> 0.2.0)

lib/gitlab_monitor/version.rb

 module GitLab
   module Monitor
-    VERSION = "3.3.0".freeze
+    VERSION = "3.3.1".freeze
   end
 end

lib/gitlab_monitor/web_exporter.rb

@@ -40,7 +40,8 @@ module GitLab
           setup_server(config[:server])
           setup_probes(config[:probes])
 
-          use MemoryKillerMiddleware, config[:server][:memory_threshold] || 1024
+          memory_threshold = (config[:server] && config[:server][:memory_threshold]) || 1024
+          use MemoryKillerMiddleware, memory_threshold
         end
 
         def setup_server(config)