Bluetooth Metrics: Add metric_id field for Bluetooth metrics to proto.

The incremental id is neither a device id nor a pseudo name and
therefore PII free.

Test: Build successfully
Bug: 142563377
Change-Id: I5393aee2d1940470678c3d00153a4771e5e676e1
Merged-In: I5393aee2d1940470678c3d00153a4771e5e676e1
(cherry picked from commit 71c3a5d4)