Optimize HttpClientTracer methods for invalid spans

[trask]

In particular, startScope(Span, ...) and end*(Span, ...) methods can be optimized for invalid spans.

This will be important after removing CallDepth (#907) which will then make the invalid span route more common.

[breedx-splk]

@trask Would you mind adding a little detail about what makes a span invalid and what that optimization might look like?

[trask]

@breedx-splk thx, #1840 addresses this, I'll note in that PR that it closes this issue

[mateuszrzeszutek]

@trask the PR you linked probably isn't going to be merged soon (😄 ), would you mind adding some explanation here? (Or closing this issue if it's no longer valid)

[trask]

oh ya, this is done now / no longer valid 👍