log/field.go
func Float64(key string, val float64) Field {
	return Field{
		key:        key,
		fieldType:  float64Type,
		numericVal: int64(math.Float64bits(val)),
	}
}
func Float64(key string, val float64) Field {
	return Field{
		key:        key,
		fieldType:  float64Type,
		numericVal: int64(math.Float64bits(val)),
	}
}
// Float64 adds a float64-valued key:value pair to a Span.LogFields() record