Consider marking Span.constructor protected

[lcapaldo]

Creating a span via new Span will almost never do what folks intend, especially if they have a non noop tracer. I appreciate that doing this would make the noop tracer implementation for javascript somewhat more complex, but I have seen folks using new Span for "default" parent spans (when using an actual tracer implementation), so it may be worth it to help consumers fall into the "pit of success."