Support for OpenTracing

[screwyprof]

It would be great if web3signer could have tracing support.

The important use cases / workflows would be:

• deposit payload signature request
• VEM signature request
• slashing protection triggered

This kind of tracing support will improve visibility of the end-to-end process for key events - these are sensitive processes and if things go wrong, the better our transparency, the quicker we'll be able to resolve

[non-fungible-nelson]

@beetrootkid - what do you think the priority of something like this would be?

[beetrootkid]

@non-fungible-nelson - I do think this is important. (disclosure: @screwyprof is in my team) It will tell us not only about when things go wrong but also help identify where time is being spent. As we ramp up active keys, then this might well become a vital piece of data...