Add metric for cyclomatic and/or cognitive complexity

[fniessink]

The current "Complex units" metric measures the amount of units that are too complex. This metric would measure the total cyclomatic complexity and/or the cyclomatic complexity density (complexity per LOC).

SonarQube, manual number, and random number are the possible sources. Tags would include "maintainability" and maybe "testability".

This metric should be "informative" by default.

Cognitive complixity link: https://www.sonarsource.com/resources/white-papers/cognitive-complexity.html