Merge branch 'mk/configure-server-type' into 'master'

Allow to specify which server to use

See merge request gitlab-org/gitlab-exporter!127

config/gitlab-exporter.yml.example

@@ -6,6 +6,7 @@ db_common: &db_common
 
 # Web server config
 server:
+  name: puma # cf. https://github.com/sinatra/sinatra#available-settings
   listen_address: 0.0.0.0
   listen_port: 9168
   # Maximum amount of memory to use in megabytes, after which the process is killed

lib/gitlab_exporter/web_exporter.rb

@@ -36,6 +36,8 @@ module GitLab
       end
 
       class << self
+        DEFAULT_WEB_SERVER = "webrick".freeze
+
         def setup(config)
           setup_server(config[:server])
           setup_probes(config[:probes])
@@ -52,6 +54,7 @@ module GitLab
         def setup_server(config)
           config ||= {}
 
+          set(:server, config.fetch(:name, DEFAULT_WEB_SERVER))
           set(:bind, config.fetch(:listen_address, "0.0.0.0"))
           set(:port, config.fetch(:listen_port, 9168))
         end