Reconsider requirement level of network.protocol.name

[trask]

From @mateuszrzeszutek

one thing that I haven’t implemented yet is the network.protocol.name opt-in attribute — right now we’re exporting it all the time
we’ve briefly talked about it in the last SIG meeting, I’m slightly opposed to removing it — to me the network.protocol.(name|version) make a pair, and sending just the version seems incomplete

and also we have some legacy HTTP clients that still technically support the deprecated protocols

network.protocol.name was changed from recommended to opt-in because:

• having default value (http) on a non-required attribute made it ambiguous whether the instrumentation just doesn't capture the attribute or whether it does capture it but it was the default value (Attributes should only have default values if supplying the attribute is (otherwise) required #385)
• removing the default value (http) would result in a high noise to value attribute stamped on all HTTP spans

I think we may be able to address this by making network.protocol.name conditionally required when network.protocol.version is provided, and making http the default value (only) when network.protocol.version is provided.