code: func SetFields(c context.Context, fields Fields) context.Context {
  	return context.WithValue(c, fieldsKey, GetFields(c).Copy(fields))
  }
docs: // SetFields adds the additional fields as context for the current Logger. The
  // display of these fields depends on the implementation of the Logger. The
  // new context will contain the combination of its current Fields, updated with
  // the new ones (see Fields.Copy). Specifying the new fields as nil will
  // clear the currently set fields.