We should support users specifying the http.route and error.type attribute on automatically instrumented HTTP metrics.

To do this, I propose we copy the relevant attributes off the associated span on span completion. I suggest we do this as follows:

• The span for the HTTP server is created in Rust (as an OtelSpan struct) and is associated with the request.
• The span object can be exposed to JS with an op, similar to how op_http_get_request_method_and_url works. It takes the external for the HttpRequest, and returns a OtelSpan. The span is then put into AsyncContext.
• The user can now update the span (adding http.route, error.type, etc).
• When the response is finalized, we read the attributes relevant to the metric we'd like to record, in Rust, from the span.