MockTracer._allocSpan is private and can't be overwritten

[oranoran]

MockTracer._allocSpan is private and can't be overwritten - so trace can't be fully customized. Should be protected.

[yurishkuro]

Not that I am particularly opposed to this change, but making the method protected extends the API surface. Please elaborate on what you are doing with mock tracer that requires you to extend it.

[ithildir-paypal]

It appears https://github.com/opentracing/opentracing-javascript/blob/master/src/mock_tracer/mock_tracer.ts#L20-L22 explicitly talks about overriding _allocSpan, but, since it's private, this is not possible.