econds of the requested clock source.

The `source` argument selects the clock source to use, when not specified the default is `:realtime`:
- :realtime: Return the real (i.e., wall-clock) time. This clock is affected by discontinuous jumps in the system time
- :monotonic: Return the number of whole + fractional seconds since some fixed point in time. The clock is guaranteed to be non-decreasing in real time.
- :cputime: Return the CPU time consumed by this process (i.e. all threads in the process)
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