Unable to implement custom loggers

[polastre]

Two of your types don't allow for implementing custom loggers outside of your package because the fields are internal to the package.

In particular:

type Valuer interface {
	getValue() interface{}
}

If this was instead:

type Valuer interface {
	Value() interface{}
}

then other packages could implement loggers that can access the value of the Valuer. As it is currently written, only loggers you define in your log package can access the value of a Valuer.

I get that you're using Valuer interface to get around passing any, but I think passing any would be better. Instead you've redefined any to be a struct, when newer versions of go already support any aliased to interface{}.

Additionally, LoggedError has internal fields and is not an interface. Recommend that this be changed from a struct to an interface.

type LoggedError struct {
	err error
}

func (l LoggedError) Err() error {
	return l.err
}

func (l LoggedError) Nil() error {
	return nil
}

instead consider:

type LoggedError interface {
	Err() error
	Nil() error
}

type LoggedErrorImpl struct {
	err error
}

func (l LoggedErrorImpl) Err() error {
	return l.err
}

func (l LoggedErrorImpl) Nil() error {
	return nil
}

I'm also not sure I understand the purpose of a Nil() func that always returns nil.