Document env variable for setting max. message metadata

Author: AramAlsabti

source/installation-guide/installation-guide.rst

@@ -229,6 +229,8 @@ OS2IoT-backend takes several environment variables as configuration, if these ar
 +-------------------------------+------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------+
 | LOG_LEVEL                     | Minimum Log Level. Levels ordered from high to low are: 'log', 'error', 'warn', 'debug', 'verbose'   | :code:`debug`                                                                           |
 +-------------------------------+------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------+
+| METADATA_SAVED_COUNT          | Maximum amount of message metadata to store from an IoT device                                       | :code:`20`                                                                              |
++-------------------------------+------------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------+
 
 Logs levels
 """""""""""""""