tracing-actix-web-mozlog: Remove request.summary fields from tracing span

[mythmon]

Currently many fields that are recommended for mozlog's request.summary event are also including the general tracing span values. This makes it harder for downstream consumers to control what is in their logs, and it unneededly bloats each log line in our current JSON formatter (which includes fields from all parent spans in each log line).

We should remove unneeded fields from this span, especially path and agent. The request ID field (rid) should stay in place, so that events can be tied together.