func Duration(key string, val time.Duration) Field {
	return Field{
		key:        key,
		fieldType:  durationType,
		numericVal: int64(val),
	}
}
Uint64 adds a uint64-valued key:value pair to a Span.LogFields() record
package fields
import (
	"fmt"
	"math"
	"time"
)
type fieldType int
const (
	stringType fieldType = iota
	boolType
	int64Type
	uint64Type
	durationType
	float64Type
)
type Field struct {
	key        string
	fieldType  fieldType
	numericVal int64
	stringVal  string
}
func String(key, val string) Field {
	return Field{
		key:       key,
		fieldType: stringType,
		stringVal: val,
	}
}
func Bool(key string, val bool) Field {
	var numericVal int64
	if val {
		numericVal = 1
	}
	return Field{
		key:        key,
		fieldType:  boolType,
		numericVal: numericVal,
	}
}
func Int64(key string, val int64) Field {
	return Field{
		key:        key,
		fieldType:  int64Type,
		numericVal: val,
	}
}
func Uint64(key string, val uint64) Field {
	return Field{
		key:        key,
		fieldType:  uint64Type,
		numericVal: int64(val),
	}
}