increase correctness of open conn count (#738)

Currently, it appears that GetOpenConnectionsCount underreports the
count by 1 after Shutdown has been called. This leads to a confusing
value when servers are gracefully terminating. For instance, if a server
is stuck in graceful termination, and there is one open connection
keeping it alive, GetOpenConnectionsCount would report zero.

This fix removes the decrement while the server is shutting down. It is
not perfect due to the use of two sequential atomic loads, but in the
common case it should return a more correct value overall.

server.go

@@ -1822,7 +1822,15 @@ func (s *Server) GetCurrentConcurrency() uint32 {
 //
 // This function is intended be used by monitoring systems
 func (s *Server) GetOpenConnectionsCount() int32 {
-	return atomic.LoadInt32(&s.open) - 1
+	if atomic.LoadInt32(&s.stop) == 0 {
+		// Decrement by one to avoid reporting the extra open value that gets
+		// counted while the server is listening.
+		return atomic.LoadInt32(&s.open) - 1
+	}
+	// This is not perfect, because s.stop could have changed to zero
+	// before we load the value of s.open. However, in the common case
+	// this avoids underreporting open connections by 1 during server shutdown.
+	return atomic.LoadInt32(&s.open)
 }
 
 func (s *Server) getConcurrency() int {