PRS and ERS don't promote replicas taking backups

changelog/22.0/22.0.0/changelog.md

@@ -0,0 +1,5 @@
+# Changelog of Vitess v22.0.0
+
+### Enhancement 
+#### Cluster management
+ * Prefer not promoting replicas that are taking backups [#16997](https://github.com/vitessio/vitess/pull/16997)

changelog/22.0/22.0.0/summary.md

@@ -4,6 +4,7 @@
 
 - **[Major Changes](#major-changes)**
   - **[RPC Changes](#rpc-changes)**
+  - **[Prefer not promoting a replica that is currently taking a backup](#reparents-prefer-not-backing-up)**
 
 
 ## <a id="major-changes"/>Major Changes</a>
@@ -13,3 +14,15 @@
 These are the RPC changes made in this release - 
 
 1. `GetTransactionInfo` RPC has been added to both `VtctldServer`, and `TabletManagerClient` interface. These RPCs are used to fecilitate the users in reading the state of an unresolved distributed transaction. This can be useful in debugging what went wrong and how to fix the problem.
+
+### <a id="reparents-prefer-not-backing-up"/>Prefer not promoting a replica that is currently taking a backup
+
+Emergency reparents now prefer not promoting replicas that are currently taking backups with a backup engine other than
+`builtin`. Note that if there's only one suitable replica to promote, and it is taking a backup, it will still be
+promoted.
+
+For planned reparents, hosts taking backups with a backup engine other than `builtin` are filtered out of the list of
+valid candidates. This means they will never get promoted - not even if there's no other candidates.
+
+Note that behavior for `builtin` backups remains unchanged: a replica that is currently taking a `builtin` backup will
+never be promoted, neither by planned nor by emergency reparents.