Report event.count and span.duration.summary for service destination metrics

Currently, we use `span.destination.service.resource.{sum.us,count}` and `event.outcome` to determine latency, throughput and failure rate. This is somewhat inconsistent with (service) transaction metrics where we are able to use `transaction.duration.summary` and `event.success_count` to determine those metrics. Similarly, for service destination metrics, we could use `span.duration.summary` and `event.success_count`. This would make it easier for the APM app to build shared queries and for users to build visualisations.