floss: Introduce metrics topshim and Floss metrics skeleton

The patch introduces the metrics topshim layer to help propagate
metrics events from Rust.

This patch also proposed the implementation of the metrics for Floss.
The events generated by the rust layer will land at gd/metrics through
the CXX bridge. OS-specific implementation will happen under
gd/metrics/$TARGET_OS.

Bug: 232098615
Tag: #floss
Test: ./build.py --target test

Change-Id: If19ce5990b20077236dcf8e3ff49207c8a57d9c5