Customize the Logger metadata key name

[crbelaus]

In certain scenarios we may want to use the Plug.RequestID plug multiple times with different headers or assigns.
While this is possible, the issue is that every call adds a :request_id key to the Logger metadata. When using the plug multiple times, the last call will override the metadata of the previous one.

defmodule MyAppWeb.Endpoint do
  use Phoenix.Endpoint, otp_app: :my_app

  plug Plug.RequestId
  # This works, but overwrites the previous `:request_id` value in the logger metadata
  plug Plug.RequestId, http_header: "x-correlation-id", assign_as: :correlation_id
end

This commit adds a new :logger_metadata_key option that allows users to customize how the request ID will be added to the logger metadata. If not provided, this option defaults to :request_id for backwards compatibility.

Previous code could be modified as:

defmodule MyAppWeb.Endpoint do
  use Phoenix.Endpoint, otp_app: :my_app

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

[crbelaus]

It is worth noting that I initially planned to use the same key as the :assign_as option and fall back to :request_id if none was provided.

At the end I decided against it because it could break existing applications that were using a custom :assign_as option but relying on the :request_id key in the logger metadata. Configuring it as a separate option does not cause this problem.

[whatyouhide]

I think this is a nice addition. 👍 Left a couple comments first.

[whatyouhide]

I would name this :logger_metadata_key.

[whatyouhide]

Suggested change

	the request id will be stored as `:request_id`.
	the request ID Logger metadata key defaults to `:request_id`.

[whatyouhide]

Suggested change

	plug Plug.RequestId, log_as: :my_request_id
	plug Plug.RequestId, log_as: :my_request_id
	*Available since v1.18.0*.

[crbelaus]

Thanks for the feedback @whatyouhide! 🙇‍♂️

I've applied your suggestions and this is ready for another review.

[josevalim]

💚 💙 💜 💛 ❤️

[gmile]

Thanks for this addition 🎉 It just so happened that I had to roll my own plug to do this a few days ago, roughly at the time when this was being submitted :) Crazy timing!

FWIW, here's my use-case for this. Using this makes it possible to link all HTTP requests made by browser during a test in Playwright - with logs:

1. in Playwright (see testInfo and setExtraHTTPHeaders):

   // in Playwright
   context = browser.newContext({ extraHTTPHeaders: {'x-test-id': testInfo.testId} })

2. in Plug:

   plug Plug.RequestId, http_header: "x-test-id", assign_as: :test_id, logger_metadata_key: :test_id

3. in logger config:

   config :logger, :default_formatter,
     format: "$time $metadata[$level] $message\n",
     metadata: [:request_id, :test_id]

In other words, after running a Playwright suite, it's now possible to filter out all entries logged during a specific test.