Provide placeholder expander for 3rd party plugins

**Describe the solution you'd like**

Providing placeholder expander for 3rd party plugins.

**Describe alternatives you've considered**

none

**Additional context**

https://github.com/fluent/fluent-plugin-prometheus/pull/129

> btw I'm wondering long time that fluentd officially provides the way to expand placeholder by messages to 3rd party plugins or not. Otherwise every plugins coping this implementation or implement their own exapander with a different UI.

An author of 3rd party plugins needs to implement their own placeholder expander.