Provide required OpenTelemetry span attributes

I'm pathfinding OpenTelemetry testing with transactions currently.  I'll collate any bugs found here:

• request_encoding span is missing db.system required by the SDK-RFC.
• Because transactions uses KV at a low-level, we don't get the mutate_in and request_encoding spans included.  I'm undecided if mutate_in is required (arguably the transaction operation replaces it), but request_encoding should certainly be there.
• The metrics we output aren't very usable currently.  Each app will output its own cumulative version of the db.couchbase.operations metric, starting from 0.  We need to insert some uniqueness into the attributes so that metrics consumers can display and aggregate these disparate versions.
• "unknown_service:java" appears as the service name for all our spans.  We probably want to allow customising this.
  • After further digging, this can only be set at the TracerProvider level, outside of our control.