puppet: Allow enabling "stats sizes" in memcached.

puppet/zulip/manifests/profile/memcached.pp

@@ -18,6 +18,7 @@ class zulip::profile::memcached {
   package { $memcached_packages: ensure => installed }
 
   $memcached_max_item_size = zulipconf('memcached', 'max_item_size', '1m')
+  $memcached_size_reporting = zulipconf('memcached', 'size_reporting', false)
   $memcached_memory = zulipconf('memcached', 'memory', $zulip::common::total_memory_mb / 8)
   file { '/etc/sasl2':
     ensure => directory,

puppet/zulip/templates/memcached.conf.template.erb

@@ -55,3 +55,7 @@ logfile /var/log/memcached.log
 # Larger max-item-size
 -I <%= @memcached_max_item_size %>
 <% end -%>
+<%- if @memcached_size_reporting %>
+# Enable `stats sizes` (see https://github.com/memcached/memcached/blob/master/doc/protocol.txt)
+-o track_sizes
+<% end -%>