This plugin was created by @yaauie: https://github.com/yaauie/logstash-filter-memcached

From its docs:

==== Description

The Memcached filter provides integration with external data in Memcached.

It currently provides the following facilities:

• get: get values for one or more memcached keys and inject them into the event at the provided paths
• set: set values from the event to the corresponding memcached keys