kvserver: segregate replica metrics in the replicateQueue

This commit adds separate metrics for tracking adding/removing/rebalancing
of voting/non-voting replicas. For instance, the add replica count is an
aggregation of voter and non-voter addition counts, which are now also
separately tracked. Also added a metric for tracking adding and removing
decommissioning replicas.

Fixes #60694
Release justification: additive change to add metrics for existing
functionality
Release note: None

pkg/kv/kvserver/allocator.go

@@ -185,6 +185,17 @@ const (
 	nonVoterTarget
 )
 
+// replicaStatus represents whether a replica is currently alive,
+// dead or decommissioning.
+type replicaStatus int
+
+const (
+	_ replicaStatus = iota
+	alive
+	dead
+	decommissioning
+)
+
 // AddChangeType returns the roachpb.ReplicaChangeType corresponding to the
 // given targetReplicaType.
 //