*
* @param string|bool $metric Metric to use, with the following possibilities:
* - user: User CPU time (without system calls)
* - cpu: Total CPU time (user and system calls)
* - wall (or any other string): elapsed time
* - false (default): will fall back to default metric
*
* @param string|bool $metric Metric to use, with the following possibilities:
* - user: User CPU time (without system calls)
* - cpu: Total CPU time (user and system calls)
* - wall (or any other string): elapsed time
* - false (default): will fall back to default metric