function::thread_indent - returns an amount of space with the current task
the amount of space added/removed for each call
This function returns a string with appropriate indentation for a thread. Call
it with a small positive or matching negative delta. If this is the real
outermost, initial level of indentation, then the function resets the relative
timestamp base to zero. The timestamp is as per provided by the
__indent_timestamp function, which by default measures microseconds.