Fluentd v0.14 provides Counter API for purposes below:

• to provide counters with increment/decrement methods which works efficiently
• to support to persistent counting data using plugin storages
• to provide shared counter between processes

It's not necessary to make it pluggable, because it only has some variant:

• visible from a process
• shared with multi processes

This API is very useful to implement counter plugins (e.g., flowcounter, datacounter, ...) and also to implement to get metrics of Fluentd itself. Persistency can be delegated into Plugin Storage implementations.