Add struct context

[alovak]

Add the StructContext function that allows to log struct fields automatically by using tags.

import "github.com/moov-io/base/log"

// Define a struct with log tags
type User struct {
    ID       int    `log:"id"`
    Username string `log:"username"`
    Email    string `log:"email,omitempty"` // won't be logged if empty
    Address  Address `log:"address"` // nested struct must have log tag
    Hidden   string // no log tag, won't be logged
}

//...
// Log with struct context
logger.With(log.StructContext(user)).Info().Log("User logged in")

[adamdecaf]

Cool idea. It shouldn't be too slow for small/medium structs. This can go in the next minor release.

[alovak]

Now we can see what goes to logs and what to traces/spans :)