http server, before request, should start a new request context

[spellr]

Right now the server can use RequestContextManager to create a new context when it get a request. That way, the context's span will be used by the http clients it uses and the trace will be passed to the servers that it calls. But no reason it won't happen in http_server's before_request.

That way, when a server sends request to another server using an http client, the same trace of the incoming request will be used by the next servers, and the server doesn't need to create the context itself.

[yurishkuro]

RequestContextManager is going to be deprecated once PR #54 is merged.

I am not sure what exactly you're proposing.