update changelog

Signed-off-by: yeya24 <benye@amazon.com>

Author: yeya24

CHANGELOG.md

@@ -82,6 +82,7 @@
 * [ENHANCEMENT] Implement versioned transactions for writes to DynamoDB ring. #6986
 * [ENHANCEMENT] Add source metadata to requests(api vs ruler) #6947
 * [ENHANCEMENT] Add new metric `cortex_discarded_series` and `cortex_discarded_series_per_labelset` to track number of series that have a discarded sample. #6995
+* [ENHANCEMENT] Ingester: Add `cortex_ingester_tsdb_head_stale_series` metric to keep track of number of stale series on head. #7071
 * [BUGFIX] Ingester: Avoid error or early throttling when READONLY ingesters are present in the ring #6517
 * [BUGFIX] Ingester: Fix labelset data race condition. #6573
 * [BUGFIX] Compactor: Cleaner should not put deletion marker for blocks with no-compact marker. #6576