SugaredLogger: Turn error into zap.Error

[abhinav]

Is your feature request related to a problem? Please describe.

With Zap's sugared logger, if you provide a plain error object to Infow/Errorw/etc.,
it does not desugar the error into a zap.Field.

log.Warnw("something happened", err) // bad

log.Warnw("something happened", "err", err) // good
// desugars to
log.Desugar().Warn("something happened", zap.NamedError("err", err))

This is inconvenient for the majority case where there will be only one error.

Describe the solution you'd like

We should turn the first case into,

log.Desugar().Warn("something happened", zap.Error(err))

We could even be smart about it in desugarFields
and make sure there's only one instance of "err".
Others have to have a field name.

Describe alternatives you've considered

Add "err" next to every err argument.

Is this a breaking change?
No

[craigpastro]

I would like to try giving this a shot if that's okay?

[abhinav]

Sure, @craigpastro!