Add subtext to changelog

.chloggen/elasticsearchexporter_optimized-json-encoding.yaml

@@ -15,7 +15,7 @@ issues: [37032]
 # (Optional) One or more lines of additional information to render under the primary note.
 # These lines will be padded with 2 spaces and then inserted directly into the document.
 # Use pipe (|) for multiline entries.
-subtext:
+subtext: Increases throughput for metrics by 2x and for logs and traces by 3x
 
 # If your change doesn't affect end users or the exported elements of any package,
 # you should instead start your pull request title with [chore] or use the "Skip Changelog" label.