Surface gemini-cli token usage

Updates the code under //agents/testing/ to surface gemini-cli's token
usage during tests. This currently just gets logged when reporting
results, but will eventually be uploaded to either ResultDB or the
performance dashboard.

Bug: 449818513
Change-Id: I8fffc499e0ec69f60758541b377bb50fc56a94b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7038209
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529726}
NOKEYCHECK=True
GitOrigin-RevId: 64e9189f9a07c514d8a71c407386e8b2af24449b