changelog: add entry for datastream changes in 8.10 (#11914)

(cherry picked from commit bc72990)

changelogs/8.10.asciidoc

@@ -46,6 +46,7 @@ Fix tail-based sampling discarding low throughput and low sample rate traces {pu
 ==== Breaking Changes
 - Aggregated metrics now consider global labels to be part of a service's identity, and high cardinality global labels may cause services to be obscured {pull}11386[11386]
 - Event protobuf encoding for tail-based sampling changed (to a more efficient encoding) for event timestamp and duration {pull}11386[11386]
+- Use object notation for data_stream fields {pull}11311[11311]
 
 [float]
 ==== Bug fixes