Add CHANGELOG.md entry for PR 2571 (#2773)

Signed-off-by: Peter Štibraný <[email protected]>

Author: pstibrany

CHANGELOG.md

@@ -52,6 +52,7 @@
 * [CHANGE] Memberlist KV client is no longer considered experimental. #2725
 * [CHANGE] Change target flag for purger from `data-purger` to `purger` and make delete request cancellation duration configurable. #2760
 * [CHANGE] Removed `-store.fullsize-chunks` option which was undocumented and unused (it broke ingester hand-overs). #2656
+* [CHANGE] Query with no metric name that has previously resulted in HTTP status code 500 now returns status code 422 instead. #2571
 * [FEATURE] TLS config options added for GRPC clients in Querier (Query-frontend client & Ingester client), Ruler, Store Gateway, as well as HTTP client in Config store client. #2502
 * [FEATURE] The flag `-frontend.max-cache-freshness` is now supported within the limits overrides, to specify per-tenant max cache freshness values. The corresponding YAML config parameter has been changed from `results_cache.max_freshness` to `limits_config.max_cache_freshness`. The legacy YAML config parameter (`results_cache.max_freshness`) will continue to be supported till Cortex release `v1.4.0`. #2609
 * [FEATURE] Experimental gRPC Store: Added support to 3rd parties index and chunk stores using gRPC client/server plugin mechanism. #2220