#399 makes server.port conditionally required

If not default (80 for http scheme, 443 for https).

with the assumption that

we expect server.port to be available to all HTTP server instrumentation.

but this seems inconsistent with the server.address requirement level of recommended.

I think it may make sense for server.address to be required on HTTP server spans also, given the benefit of backends being able to reconstruct the original URL (not withstanding bogus Host headers). Note however that this would reverse #111.