raft: ComputeLeadSupportUntil on every leader tick

This commit calls ComputeLeadSupportUntil on every leader tick rather
than on every leader step. If the range is idle, the leader won't have
anything to step on, but it will keep ticking.

References: #139072

Release Note: None

pkg/raft/raft.go

@@ -1210,6 +1210,9 @@ func (r *raft) tickElection() {
 func (r *raft) tickHeartbeat() {
 	assertTrue(r.state == pb.StateLeader, "tickHeartbeat called by non-leader")
 
+	// Compute the LeadSupportUntil on every tick.
+	r.fortificationTracker.ComputeLeadSupportUntil(r.state)
+
 	// Check if we intended to step down. If so, step down if it's safe to do so.
 	// Otherwise, continue doing leader things.
 	if r.fortificationTracker.SteppingDown() && r.fortificationTracker.CanDefortify() {
@@ -1841,9 +1844,6 @@ func (r *raft) logMsgHigherTerm(m pb.Message, suffix redact.SafeString) {
 type stepFunc func(r *raft, m pb.Message) error
 
 func stepLeader(r *raft, m pb.Message) error {
-	// Compute the LeadSupportUntil on every tick.
-	r.fortificationTracker.ComputeLeadSupportUntil(r.state)
-
 	// These message types do not require any progress for m.From.
 	switch m.Type {
 	case pb.MsgBeat: