puppet: Allow enabling "stats sizes" in memcached.

docs/production/system-configuration.md

@@ -306,6 +306,12 @@ Override the maximum size that an item in memcached can store. This defaults to
 1m; adjusting it should only be necessary if your Zulip server has organizations
 which have more than 20k users.
 
+#### `size_reporting`
+
+Set to a true value to enable object size reporting in memcached. This incurs a
+small overhead for every store or delete operation, but allows a
+memcached_exporter to report precise item size distribution.
+
 ### `[loadbalancer]`
 
 #### `ips`

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 -%>