Count BFD session state changes from UP to DOWN (#2268)

Signed-off-by: Chikkegowda Chikkaiah <cchikkai@cisco.com>

Author: chikkaiah-work

inc/saibfd.h

@@ -589,7 +589,10 @@ typedef enum _sai_bfd_session_stat_t
     SAI_BFD_SESSION_STAT_OUT_PACKETS,
 
     /** Packet Drop stat count */
-    SAI_BFD_SESSION_STAT_DROP_PACKETS
+    SAI_BFD_SESSION_STAT_DROP_PACKETS,
+
+    /** Count transitions where the session state changes from UP to DOWN */
+    SAI_BFD_SESSION_STAT_SESSION_UP2DOWN_TRANSITIONS
 
 } sai_bfd_session_stat_t;