Duplicating attributes from dispatch_to_server to improve debugging

Most, perhaps all, current tracing systems don't allow performing queries across spans. E.g. there is no way of finding dispatch_to_server spans where <some attribute in a parent span> == <some value>. One useful query along those lines would be checking the server_duration times against different buckets.

After discussions with one of the tracing providers, we understand there are two solutions to this: duplicate some attributes between spans, or use OpenTelemetry 'baggage'. In practice, baggage does not seem useful: it is unclear/undocumented how to attach it through the API and, many (perhaps all) tracing systems do not appear to expose it.

So, instead we will duplicate some useful attributes between the dispatch_to_server and top-level KV span.  A full justification for which attributes is provided here: https://docs.google.com/document/d/1BAPS8bPMv8-4FPIdysgpxEsKrUgd595EAGOU-_nXHRY/edit?disco=AAAALpOPEZU