filter: lua: add description about 'type_array_key' (#538)

Signed-off-by: Takahiro Yamashita <nokute78@gmail.com>

Author: nokute78

pipeline/filters/lua.md

@@ -16,6 +16,7 @@ The plugin supports the following configuration parameters:
 | script | Path to the Lua script that will be used. |
 | call | Lua function name that will be triggered to do filtering. It's assumed that the function is declared inside the Script defined above. |
 | type\_int\_key | If these keys are matched, the fields are converted to integer. If more than one key, delimit by space. Note that starting from Fluent Bit v1.6 integer data types are preserved and not converted to double as in previous versions. |
+| type\_array\_key| If these keys are matched, the fields are handled as array. If more than one key, delimit by space. It is useful the array can be empty. |
 | protected\_mode | If enabled, Lua script will be executed in protected mode. It prevents to crash when invalid Lua script is executed. Default is true. |
 | time\_as\_table | By default when the Lua script is invoked, the record timestamp is passed as a Floating number which might lead to loss precision when the data is converted back. If you desire timestamp precision enabling this option will pass the timestamp as a Lua table with keys `sec` for seconds since epoch and `nsec` for nanoseconds. |