news: add entry for axoflow#365

Signed-off-by: Attila Szakacs <attila.szakacs@axoflow.com>

news/fx-feature-365.md

@@ -0,0 +1,12 @@
+`metric_labels()`: Added a new dict-like type to store metric labels directly.
+
+This dict converts the key-values to metric labels on the spot,
+so when it is used in multiple `update_metric()` function calls,
+no re-rendering takes place, which greatly improves performance.
+
+The stored labels are sorted alphabetically.
+
+Be aware, that this is a list of key-value pairs, meaning key collisions
+are not detected. Use the `dedup_metrics_labels()` function to deduplicate
+labels. However, this takes CPU time, so if possible, make sure not to
+insert a key multiple times so `dedup_metrics_labels()` can be omitted.