support to add a flag for throttled records instead of dropping it

[Gtharan10]

Hi,

For our application we needs an additional custom field in the record to indicate throttling instead of discarding it. Based on that we will do post processing things on that record. This new field should be named "add_field," and the flag associated with it will determine whether the record should be dropped or modified, allowing it to continue with further processing.

   <filter syslog.**>
        @type throttle
        group_key  type
        group_bucket_period_s  60
        group_bucket_limit  5000
        group_reset_rate_s  -1
        add_field  throttled  # added acknowledgment like yes for this field.
        proceed_further  true   #defaults as false.
   </filter> 

We would appreciate your input on this.