[check-licenses] Introduce core metrics package

Adds a new, centralized `tools/check-licenses/metrics` package that provides a high-performance, thread-safe, and dimensional observability framework for `check-licenses`.

This paves the way for migrating away from the decentralized metrics currently spread across individual packages.

Test: fx test check-licenses
Change-Id: I9aea1b6f08e000ef5bb6e92c8d0d8bad9231e403
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1570893
Commit-Queue: Jerry Belton <jcecil@google.com>
Reviewed-by: Jay Zhuang <jayzhuang@google.com>
Reviewed-by: Jiaming Li <lijiaming@google.com>