Only decrement server_interactive_sessions_total once per session

`session.Close` can get called multiple times, from different deferred
cleanups. The associated metric decrement should only happen on the
first call, to map 1:1 with increments.
Without this, we could end up with negative
`server_interactive_sessions_total` counts.

Fixes #3755

lib/srv/sess.go

@@ -49,7 +49,7 @@ const (
 var (
 	serverSessions = prometheus.NewGauge(
 		prometheus.GaugeOpts{
-			Name: "server_interactive_sessions_total",
+			Name: teleport.MetricServerInteractiveSessions,
 			Help: "Number of active sessions",
 		},
 	)
@@ -563,8 +563,8 @@ func (s *session) PID() int {
 
 // Close ends the active session forcing all clients to disconnect and freeing all resources
 func (s *session) Close() error {
-	serverSessions.Dec()
 	s.closeOnce.Do(func() {
+		serverSessions.Dec()
 		// closing needs to happen asynchronously because the last client
 		// (session writer) will try to close this session, causing a deadlock
 		// because of closeOnce