adjust metrics methods for calculating CPU time to match specs, add new specs for fixnum case

What does this MR do?

I noticed an inconsistency with the intended output from

It states that it should return a Float, and the spec expects a float, however :millisecond will return a Fixnum. I just set the precondition of the argument to :float_millisecond per the documentation on Process

Are there points in the code the reviewer needs to double check?

I've written specs to test when the :millisecond precision param is passed into the methods, other than that nothing really.

Why was this MR needed?

The specs fail due to expecting a Float. I'm testing on Ubuntu 16.04

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Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

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