Add required conditions for Logger's metadata to be print (#13777)

lib/logger/lib/logger/formatter.ex

@@ -74,6 +74,21 @@ defmodule Logger.Formatter do
   to the current process. The user can configure the backend to choose
   which metadata it wants to print and it will replace the `$metadata`
   value.
+
+  > #### When is user metadata printed? {: .warning}
+  >
+  > User's metadata must meet one of the following conditions to be printed:
+  >
+  >   * Be a string (`is_binary/1`)
+  >   * Be a number (either `is_integer/1` or `is_float/1`)
+  >   * Be a PID
+  >   * Be an atom
+  >   * Be a reference
+  >   * Be a port
+  >   * Implement the `String.Chars` protocol
+  >
+  > If none of the conditions above are `true`, the given metadata get
+  > discarded.
   """
 
   @type date :: {1970..10000, 1..12, 1..31}