[MONIT-4106] values/metrics/fluentbit: add default luaScripts value

In commit c6c3db28 we added support for lua scripts provided via user
values.

This commit adds a default value and documents this in the values file.

Reported-at: https://its.cern.ch/jira/browse/MONIT-4106


Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@gmail.com>
Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@cern.ch>

values.yaml

@@ -382,3 +382,14 @@ logs:
     extraVolumes: []
     ## -- extra volumes to mount in the fluentbits, can be used to scrape metrics from pvcs
     extraVolumeMounts: []
+
+    # These scripts are available in the fluentbit /fluent-bit/etc/scripts path.
+    # Include your lua scripts in the following format:
+    # luaScripts:
+    #  my_lua_script.lua: |
+    #   function my_function(tag, timestamp, record)
+    #     // Do something...
+    #     return 2, timestamp, record
+    #   end
+    #  my_other_lua_script.lua: ...
+    luaScripts: {}