Logger.log

You're seeing just the macro log, go back to Logger module for more information.
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log(level, message_or_fun, metadata \\ [])

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Logs a message with the given level.

Returns :ok.

The macros debug/2, info/2, notice/2, warning/2, error/2, critical/2, alert/2, and emergency/2 are preferred over this macro as they can automatically eliminate the call to Logger altogether at compile time if desired (see the documentation for the Logger module).