Protocol for encoding data types.

Example:

defmodule User do
  defstruct [:email, :name, :id]

  defimpl LogfmtEx.ValueEncoder do
    def encode(user), do: to_string(user.id)
  end
end

If there is no protocol for a data type passed as metadata, then encoding will fall back to the String.Chars protocol. If that protocol isn't specified either, the formatter will fall back to Kernel.inspect/1.

Note that the algebra documents produced by Kernel.inspect/1 don't lend themselves to logfmt - this fallback is provided to minimize the chance that the formatter fails, instead making a "best effort" at producing usable output. It is recommended to implement either the LogfmtEx.ValueEncoder or String.Chars protocol for any data types that might find their way into your logs.