src/client/pps.go
func NewUnionInput(input ...*pps.Input) *pps.Input {
	return &pps.Input{
		Union: input,
	}
}
func NewUnionInput(input ...*pps.Input) *pps.Input {
	return &pps.Input{
		Union: input,
	}
}
// NewUnionInput returns an input which is the union of other inputs. That
// means that all datums from any of the inputs will be seen individually by
// the job / pipeline.