Add attribute for classifying requests to root spans

[sissbruecker]

Currently request handler instrumentations add spans for their execution, and some of them change the root span name. Depending on the used monitoring software, both can be kind of hard to search for if you are interested in all requests of a specific type.

In order to classify types of requests it would be useful to have an attribute on the root span that could be filtered by. This attribute could be set by individual request handler instrumentations, if they determine that the request was actually handled by the handler. This could either be a vaadin.request.handler which could contain the request handler class name, or a more abstract vaadin.request.type which contains a classification of the request, for example rpc, static, stream-resource.