feat(request_id): add custom Logger's metadata option

[georgeguimaraes]

Add a way to configure the Logger metadata for Plug.RequestId.

Why? I need to use RequestId as

plug Plug.RequestId, http_header: "x-correlation-id", assign_as: :correlation_id

and then it feels weird to have request_id in the logs, don't you think?

With this PR, we'd have

plug Plug.RequestId, http_header: "x-correlation-id", assign_as: :correlation_id, metadata_as: :correlation_id

[josevalim]

Given the plug is called RequestId, I'd really expect it to at least set a request id metadata. I also don't want it to have several different options, I'd rather make it easier to roll your own. What if we make the get_request_id(conn, header) function public? So you could build your own? Then we might even deprecate assign_as :D

[georgeguimaraes]

nah, I don't think exposing get_request_id is worth it. One can have the exact same effect accessing conn.assigns[:request_id].

So, for me, if you don't accept this PR, I'm gonna create a new Plug in my app that only sets Logger.metadata with the key I want and that content.

Too hacky?

[josevalim]

So, for me, if you don't accept this PR, I'm gonna create a new Plug in my app that only sets Logger.metadata with the key I want and that content.

The idea of exposing get_request_id was exactly to make it easier for you to roll your own plug. But accessing it in assigns also works. Your call!