Compute cpuRatio values when computing the derived thread

[mstange]

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processThreadCPUDelta makes it so that the threadCPUDelta column doesn't have any null values. However, the type of the threadCPUDelta column still looks as if values could be null.
Now that we have a derived Thread type, we can change the type so that it expresses that the values can't be null. So we move the call to processThreadCPUDelta a bit earlier in the derivation pipeline so that all the types work out.

Even better, we can compute CPU ratios at this point, and not even store the "clamped and non-null cpu delta" values.

And since we're doing that, I'm changing the behavior for null cpu delta values: Rather than using the closest non-null cpu delta and spreading it over a potentially-different sample time delta, just set the cpu ratio to 1. The only known cause of null values (other than the first value, which we ignore) is the base profiler, and we want CPU percentage to be reported as 100% during the base profiler time.

Fixes #4072.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.02%. Comparing base (52f3144) to head (254026d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5288      +/-   ##
==========================================
- Coverage   86.02%   86.02%   -0.01%     
==========================================
  Files         311      312       +1     
  Lines       29791    29750      -41     
  Branches     8217     8191      -26     
==========================================
- Hits        25629    25592      -37     
+ Misses       3576     3572       -4     
  Partials      586      586              

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