Get or set log message formatter
Arguments
- formatter
 function defining how R objects are converted into a single string, eg
formatter_paste(),formatter_sprintf(),formatter_glue(),formatter_glue_or_sprintf(),formatter_logging(), default NULL- namespace
 logger namespace
- index
 index of the logger within the namespace
See also
Other log configutation functions:
log_appender(),
log_layout(),
log_threshold()
