Added note about known issue with cortex_prometheus_rule_group_last_evaluation_timestamp_seconds metric

Signed-off-by: Marco Pracucci <[email protected]>

Author: pracucci

CHANGELOG.md

@@ -3,7 +3,7 @@
 ## master / unreleased
 
 
-## 0.6.0 / 2020-01-22
+## 0.6.0 / 2020-01-28
 
 Note that the ruler flags need to be changed in this upgrade. You're moving from a single node ruler to something that might need to be sharded.
 Further, if you're using the configs service, we've upgraded the migration library and this requires some manual intervention. See full instructions below to upgrade your PostgreSQL.
@@ -70,6 +70,11 @@ Reference: https://github.com/golang-migrate/migrate/tree/master/database/postgr
 migrate  -path <absolute_path_to_cortex>/cmd/cortex/migrations -database postgres://localhost:5432/database force 2 
 ```
 
+### Known issues
+
+- The `cortex_prometheus_rule_group_last_evaluation_timestamp_seconds` metric, tracked by the ruler, is not unregistered for rule groups not being used anymore. This issue will be fixed in the next Cortex release (see [2033](https://github.com/cortexproject/cortex/issues/2033)).
+
+
 ## 0.4.0 / 2019-12-02
 
 * [CHANGE] The frontend component has been refactored to be easier to re-use. When upgrading the frontend, cache entries will be discarded and re-created with the new protobuf schema. #1734