fix(tracer): fixed create exit span with context error when current context not to be sampled

[Luckyboys]

In the tracer CreateExitSpanWithContext function, when the context is not a sample in this time, it will be return a "span type is wrong" error.

Because trying to check the span is NoopSpan when after called CreateLocalSpan function, the convertion is expected a struct value.

But actual, the CreateLocalSpan function return a Span is a interface type, a interface type is a pointer in golang.

So we need to change the convertion to expect a pointer of NoopSpan type.

fix #113

[Luckyboys]

@arugal Please review this fix, thank you