Make attributes such as http.url available before a span is started

[patsonluk]

Hi, we run into similar issue as #230 and wish to know the URL when doing sampling decision.

But it seems like with the change in https://github.com/open-telemetry/opentelemetry-auto-instr-java/pull/348/files , the http.url is still not available for example in the Servlet3Advice, the DECORATOR which adds the http.url attribute is not run until the span is started.

Wondering if it's feasible to change the DECORATOR to allow a beforeSpanStart or even change the onRequest and onConnection to take SpanBuilder and have them executed before the span is started?

Many thanks in advance!