Bluetooth: shell: Add L2CAP throughput measurement

Added l2cap-metrics command to shell application which will
collect the rx-ed L2CAP CoC throughput.

Usage (local shell):
> l2cap-register <chan>
> l2cap-metrics on

  From peer shell:
  > l2cap-connect <chan>
  > l2cap-send 10

> l2cap-metrics
l2cap rate: 119225 bps.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>