CXXCBC-736: Add dispatch spans for KV operations

[DemetrisChr]

Motivation

The Observability RFC expects having separate dispatch_to_server spans. Currently, we are recording all dispatch-level attributes on the top-level operation span.

Changes

• Add dispatch spans for KV operations, one span per server round-trip. Update the dispatch-level attribute names so they conform to the latest semantic conventions
• Update the threshold logging tracer so it can handle both the legacy top level spans that have dispatch-level attributes (temporarily, for compatibility with HTTP operations) and the new dispatch spans.
• Add "canonical" address & port fields to memd sessions, these are the default (non-external) hostname & port as they appear in the config, which are now being reported in the new server.endpoint and server.port dispatch span attributes.