Gather and report metrics for the number of channels a relay is servicing.

It would be really nice to have statistics on the min/max/average number of simultaneous channels (TCP/IP connections) a relay is servicing for a few reasons:

• If we are to add per-channel padding like in legacy/trac#16861 (moved) (or something more aggressive like people discuss), we need this to figure out how much extra bandwidth a relay will use, and possibly for parameter tuning.
• This information will be helpful in determining if someone can/should run a relay, given their environment (Eg: To figure out if their relay will end up underutilized/bad for the network due to NAT connection table/OS limitations).

I'm uncertain as to how coarse grained this will need to be, particularly for Guards. Bridges SHOULD NOT report this information.