Take over PR #3888: Add unit tests for benchmarking metrics and document metric_R2

[ayushman1210]

I am taking over #3888 PR because the original author has been inactive, and I didn't have the permissions to push directly to their fork.
This PR includes all of @tanmaydimriGSOC's original commits, plus the requested fixes to unblock it

• Removed the PEcAn.logger mock from the top of test-metrics.R.
• Refactored metric_R2.R to remove the unreliable lm() fallback, now using stats::cor()^2.
• Tightened the unit tests for metric_RMSE (expecting exactly 0) and metric_R2 (expecting NA and the standard stats::cor warning for constant inputs).

This can be merged to fully close out #3888

[divine7022]

per yesterday's discussion, please try pushing to 3888, since you haven't actually tried
don't really want to have diverging PRs on same