Logger.bare_log

You're seeing just the function bare_log, go back to Logger module for more information.
Link to this function

bare_log(level, message_or_fun, metadata \\ [])

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Specs

bare_log(
  level(),
  message() | (() -> message() | {message(), keyword()}),
  keyword()
) :: :ok

Logs a message dynamically.

Opposite to log/3, debug/2, info/2, and friends, the arguments given to bare_log/3 are always evaluated. However, you can pass anonymous functions to bare_log/3 and they will only be evaluated if there is something to be logged.