make chunk key configurable from plugins

[sabottenda]

A key name of chunk can be specified in 4th argument of BufferedOutput#emit in BufferedOutput class, but no one use this argument and no hook point for setting the value. It is good to be able to configure it from plugins like format.

If this feature is available, plugins can determine how to handle chunks depending on the chunk key. For instance, destination server is determined by the tag. I know the example can be achieved by using fluent-plugin-forest. Though I think it is easy to provide a plugin with built-in features like fluent-plugin-hash-forward.

Actually I want to set the key name from log data, but it is inefficient to call chunk_key for each message just like format. For that purpose, I think Mixin approach is better and allow format to also return a key name of chunk. How do you think about it?

[repeatedly]

Quick reply from my view point for the patch:

• We can't merge this patch because it breaks several plugins, e.g. fluent-plugin-mongo and tag_mapped like approach plugins,
• chunk_key is achieved by current key argument. I can't see the merit of this approach excluding human readable.

[sabottenda]

I read fluent-plugin-mongo implementation, so it's enough for the purpose definitely.
I'll close this PR, thanks.