Prevent races counting missed heartbeats (#49910)

Increments the `heartbeatsMissedByAuth` gauge directly instead of
keeping a local count and setting the gauge once. This fixes a
race caught in https://github.com/gravitational/teleport/actions/runs/11828850190/job/32959672995?pr=48949.

```
==================
WARNING: DATA RACE
Write at 0x00c003857da8 by goroutine 48258:
  github.com/gravitational/teleport/lib/auth.(*Server).runPeriodicOperations.func3.1()
      /__w/teleport/teleport/lib/auth/auth.go:1471 +0x84
  github.com/gravitational/teleport/lib/services.(*UnifiedResourceCache).getRange.func1.1()
      /__w/teleport/teleport/lib/services/unified_resource.go:239 +0x14d
  github.com/google/btree.(*node[go.shape.*uint8]).iterate()
      /go/pkg/mod/github.com/google/[email protected]/btree_generic.go:522 +0x62c
  github.com/google/btree.(*BTreeG[go.shape.*uint8]).AscendRange()
      /go/pkg/mod/github.com/google/[email protected]/btree_generic.go:752 +0x104
  github.com/google/btree.(*BTreeG[*github.com/gravitational/teleport/lib/services.item]).AscendRange-fm()
      <autogenerated>:1 +0x58
  github.com/gravitational/teleport/lib/services.(*UnifiedResourceCache).getRange.func1()
      /__w/teleport/teleport/lib/services/unified_resource.go:230 +0x7a8
  github.com/gravitational/teleport/lib/services.(*UnifiedResourceCache).read()
      /__w/teleport/teleport/lib/services/unified_resource.go:591 +0x2ad
  github.com/gravitational/teleport/lib/services.(*UnifiedResourceCache).getRange()
      /__w/teleport/teleport/lib/services/unified_resource.go:215 +0x2d9
  github.com/gravitational/teleport/lib/services.(*UnifiedResourceCache).IterateUnifiedResources()
      /__w/teleport/teleport/lib/services/unified_resource.go:288 +0xe6
  github.com/gravitational/teleport/lib/auth.(*Server).runPeriodicOperations.func3()
      /__w/teleport/teleport/lib/auth/auth.go:1464 +0x237

Previous read at 0x00c003857da8 by goroutine 48219:
  github.com/gravitational/teleport/lib/auth.(*Server).runPeriodicOperations.func3()
      /__w/teleport/teleport/lib/auth/auth.go:1489 +0x2d2

Goroutine 48258 (running) created at:
  github.com/gravitational/teleport/lib/auth.(*Server).runPeriodicOperations()
      /__w/teleport/teleport/lib/auth/auth.go:1460 +0xf9e
  github.com/gravitational/teleport/lib/auth.initCluster.gowrap1()
      /__w/teleport/teleport/lib/auth/init.go:594 +0x33

Goroutine 48219 (finished) created at:
  github.com/gravitational/teleport/lib/auth.(*Server).runPeriodicOperations()
      /__w/teleport/teleport/lib/auth/auth.go:1460 +0xf9e
  github.com/gravitational/teleport/lib/auth.initCluster.gowrap1()
      /__w/teleport/teleport/lib/auth/init.go:594 +0x33
==================
```

lib/auth/auth.go

@@ -1266,8 +1266,6 @@ func (a *Server) runPeriodicOperations() {
 
 	defer ticker.Stop()
 
-	missedKeepAliveCount := 0
-
 	// Prevent some periodic operations from running for dashboard tenants.
 	if !services.IsDashboard(*modules.GetModules().Features().ToProto()) {
 		ticker.Push(interval.SubInterval[periodicIntervalKey]{
@@ -1369,7 +1367,7 @@ func (a *Server) runPeriodicOperations() {
 									return false, nil
 								}
 								if services.NodeHasMissedKeepAlives(srv) {
-									missedKeepAliveCount++
+									heartbeatsMissedByAuth.Inc()
 								}
 								return false, nil
 							},
@@ -1385,9 +1383,6 @@ func (a *Server) runPeriodicOperations() {
 							break
 						}
 					}
-
-					// Update prometheus gauge
-					heartbeatsMissedByAuth.Set(float64(missedKeepAliveCount))
 				}()
 			case metricsKey:
 				go a.updateAgentMetrics()