Expose :logger's global metadata

[whatyouhide]

Part of #9465.

[josevalim]

Suggested change

	* `:metadata` - global primary metadata to be included in all log messages.
	Defaults to `[]`. See [the configuration
	section](https://www.erlang.org/doc/man/kernel_app.html#configuration) of
	the documentation for the `kernel` Erlang application.
	* `:metadata` - global primary metadata to be included in all log messages.
	Defaults to `[]`. This can be overridden at the process level with `metadata/1`
	or each on log call as desired.

[josevalim]

We should also add a note to Logger.metadata/0 that it only returns the process metadata and not the global one.

Let's also add a test in logger_test. You will already find some tests that call App.stop and Application.start(:logger) to test the level is read correctly. We can do similar for metadata.

[whatyouhide]

Done and done 👍

[josevalim]

Suggested change

	Application.get_env(:logger, :metadata, [])
	Application.fetch_env!(:logger, :metadata)

And set a default on mix.exs.

[whatyouhide]

Done!

[josevalim]

Suggested change

	:ok =
	Application.get_env(:logger, :metadata, [])
	|> Map.new()
	|> then(&Map.merge(primary_config.metadata, &1))
	|> then(&:logger.set_primary_config(:metadata, &1))
	with [_ | _] = metadata <- Application.fetch_env!(:logger, :metadata) do
	:ok = :logger.set_primary_config(:metadata, Enum.into(metadata, primary_config.metadata))
	end

So we don't bother setting up a metadata if nothing changed?

[whatyouhide]

Really nice. Done.