Review requirement level of server.address and server.port on HTTP server spans

[trask]

#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.

[lmolkova]

Thanks for digging up the context!

In #111 we discussed that server.address|port are not essential on the server side and at least some applications don't need them (the service.name may be enough especially between instances of internal microservices).

I'd prefer to keep the door open in the spec and allow users to opt-out of these attributes.

We can keep server.address recommended and add another condition for server.port to populate it if server.address is populated and when the value is not default.

This way we ensure that in the presence of address, absence of port means default value.