What metrics are be most useful to monitor during test execution?

The proper answer to this us C. You want to know if there are some defects that can be found when the test was executed. If there are no defects found, then whatever you are testing will already be perfect to use. If there are still a lot of defects, then these defects should be detected and fixed.

It will not be too useful to allow applications and software to be released to the public. If ever there are some issues that the test was unable to fix, it is best to double check the codes that were written for them. The easier that these things are detected, the better it would be.