elasticsearch/logstash — pull request #2269 by wiibaa (2014-12-19 06:29:00+00:00)
Title: Specify in doc and validate if a config item allows sprintf_format

Rework of #1517 for 1.5
Here is an idea following a discussion on logstash-user groups where someone tried to dynamicly set the host parameter of elasticsearch_http output from event field,

This proposal tag the config items that can (or cannot) use dynamic value aka sprintf under the hood.
Example done on the base filter and output