Include the user agent in the http server spans

[Symbianx]

Hello, Would be great if we could include the user agent in the spans created by the akka.http.server component. In our opentelemetry collectors we use the http.user_agent to discard traces created by liveness/readiness probes and prometheus scrapes wich helps us keep the traces database lighter.

This is something I can create a PR for, but would need a nudge in the right direction of where this should be done.