[APM] Use oldest exit span instead of newest

For discovering connections, select the oldest span instead of the newest. This increases the possiblity of the relevant transaction being indexed into Elasticsearch at query time.

x-pack/plugins/apm/server/lib/connections/get_connection_stats/get_destination_map.ts

@@ -111,7 +111,7 @@ export const getDestinationMap = ({
                   ] as const),
                   sort: [
                     {
-                      '@timestamp': 'desc' as const,
+                      '@timestamp': 'asc' as const,
                     },
                   ],
                 },