remotewrite: improve legends (#564)

* remotewrite: improve legends
* remotewrite: add count of agent replicas

---------

Co-authored-by: Herve Nicol <[email protected]>

Author: hervenicol

CHANGELOG.md

@@ -14,6 +14,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Replace Cluster ID with cluster in dashboard labels.
+- remotewrite: improve legends
+- remotewrite: add count of agent replicas
 
 ### Fixed