client.go
func OptClientTracer(tracer opentracing.Tracer) ClientOption {
	return func(options *ClientOptions) error {
		options.tracer = tracer
		return nil
	}
}
func OptClientTracer(tracer opentracing.Tracer) ClientOption {
	return func(options *ClientOptions) error {
		options.tracer = tracer
		return nil
	}
}
// OptClientTracer sets the Open Tracing tracer
// See: https://opentracing.io