Add logging guideline

[njgheorghita]

Quick, initial logging guideline - very open to change if anybody has thoughts / formatting guidelines they'd like to change / add.

[pipermerriam]

I would suggest we also go for the following display structure with fixed width "columns":

LEVEL TIMESTAMP  WHAT_HAPPENED   FIELD_A=VALUE_A FIELD_B=VALUE_B

This is largely a copy of what geth does.

[jacobkaufmann]

In my experience, it's very useful to have conventions for logging relevant data structures/fields so that querying/searching logs is less of a treasure hunt. It looks like what @pipermerriam suggested would be good. I think this would require some conventions around how we implement fmt::Display for data types as we did for Ping and Pong in #157.

[jacobkaufmann]

I also forgot to add a note about logging request IDs, but it looks like discv5 doesn't expose the request ID of a TalkRequest so I opened a PR.

[lithp]

This sounds good to me, no matter what log format we end up standardizing on these three bullets will be relevant advice.

[lithp]

Agree with @jacobkaufmann, I think the primary value of a standardized format is that it makes logs easier to grep. e.g. if you're looking for a record of all interactions with some peer you should be able to find it by grepping for node_id=$NODEID.

This is enabled by both the Ping(node_id=xxx) format and by the structured logging format @pipermerriam suggests, I'm happy with either!

[njgheorghita]

Merging this for now and tracking a more general log format standardization convo over here (#168) - would love to hear any input.