DOC-4927 Added endpoint availability metrics known issue and workaround to 7.22 release notes

Author: rrelledge

content/operate/rs/monitoring/db-availability.md

@@ -58,3 +58,17 @@ The following table shows the relationship between a database's status and avail
 | import-pending  | <span title="Available">&#x2705;</span> Available |
 | pending         | <span title="Available">&#x2705;</span> Available |
 | recovery        | <span title="Not available">:x:</span> Not available |
+
+## Known issues
+
+- RS155734: Endpoint availability metrics do not work as expected due to a calculation error. As a workaround, use this query to measure availability:
+
+    ```sh
+    endpoint_server_became_unavailable{cluster="$cluster", db="$db"} 
+    - 
+    endpoint_server_available_again{cluster="$cluster", db="$db"}
+    ```
+
+    For up: 0-2
+
+    For down: 2-1000000

content/operate/rs/release-notes/rs-7-22-releases/_index.md

@@ -95,6 +95,18 @@ The following table provides a snapshot of supported platforms as of this Redis
 
 - RS131972: Creating an ACL that contains a line break in the Cluster Manager UI can cause shard migration to fail due to ACL errors.
 
+- RS155734: Endpoint availability metrics do not work as expected due to a calculation error. As a workaround, use this query to measure availability:
+
+    ```sh
+    endpoint_server_became_unavailable{cluster="$cluster", db="$db"} 
+    - 
+    endpoint_server_available_again{cluster="$cluster", db="$db"}
+    ```
+
+    For up: 0-2
+
+    For down: 2-1000000
+
 ## Known limitations
 
 #### Upload modules before OS upgrade

content/operate/rs/release-notes/rs-7-22-releases/rs-7-22-x-tba.md

@@ -247,6 +247,18 @@ The following table shows the SHA256 checksums for the available packages:
 
 - RS131972: Creating an ACL that contains a line break in the Cluster Manager UI can cause shard migration to fail due to ACL errors.
 
+- RS155734: Endpoint availability metrics do not work as expected due to a calculation error. As a workaround, use this query to measure availability:
+
+    ```sh
+    endpoint_server_became_unavailable{cluster="$cluster", db="$db"} 
+    - 
+    endpoint_server_available_again{cluster="$cluster", db="$db"}
+    ```
+
+    For up: 0-2
+
+    For down: 2-1000000
+
 ## Known limitations
 
 #### Upload modules before OS upgrade