Move Precision, Recall and F1-score computation from `AgcManagerDirect`
to a separate function that can be tested.
Bug: webrtc:12774
Change-Id: Iba20f153a72b7f957bf938e0642055d421045c02
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/231228
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Hanna Silen <silen@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34933}