lib/discovery/ble: Export Driver.DebugString as a stat.
lib/discovery/ble: Export Driver.DebugString as a stat.

Change-Id: I4c74d0afd97053d4861355566c646065310a5d20

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref