investigate structured logging

[Geal]

sozu's logs have a common structure, especially for access logs, but there's a lot of information that's specific to the context, the application, etc. It could be interesting to check how structured logging could be applied:

• see which fields are common to each logs (because we can easily query those after processing)
• make an inventory of the different log messages
• plan how sozu could switch to structured logging

[NotBad4U]

I'll take a look at messagetemplates.org.

[NotBad4U]

Whe can take a look at this crate: https://crates.io/crates/slog