floss: metrics: start sending A2DP session metrics

Start sending the A2DP session metrics as ChromeOS structured metrics.

Bug: 300256611
Flag: EXEMPT metric change that should only affect floss
Test: atest --host net_test_btif
Test: mmma -j system
Test: emerge-${BOARD} floss
Test: build, deply and see the A2DP debug log
Change-Id: I812c0b9e2a1057805bb6274e13ef7008fa65feae