Add monitor "channel" for lcm-spy info

[kevinsbarnard]

To avoid clients (e.g., Dial) from needing to subscribe to every channel in order to monitor LCM traffic a la lcm-spy, add a "channel" with per-channel info and computed statistics:

• channel name
• fingerprint/type name (of last received message)
• number of messages (total since start)
• frequency (messages/sec)
• bandwidth (bytes/sec)

[kevinsbarnard]

TBD: frequency? lcm-spy updates every second, which would be a good rate for most applications

[carueda]

Very productive discussion today, @kevinsbarnard, and thanks for capturing this for LWS! (which supersedes https://github.com/CoMPASLab/dial/issues/59, which I'm now closing ; )

Just my opinion, but, for the Dial webapp, I would say that even every 1.5-2secs would be fine (as every fraction counts), @gtroni ?