add sequence number differences metric for accept log processing

Summary:
as title. Aside from the top level STATS_paxos_out_of_order_accept_rejected, need to introduce an extra metric to denote how far the current maxAccepted cursor is lagging behind.

Will be running the bundle in test tier to see if there is any latency bump.

Reviewed By: dyniusz

Differential Revision: D68254504

fbshipit-source-id: 5161cbcdd04607e9cfcbc7542abbe775a715d91c

Author: Dean Zhu

fb303/ExportType.h

@@ -132,6 +132,7 @@ struct QuantileConsts {
 
 struct SlidingWindowPeriodConsts {
   static constexpr const std::array<size_t, 1> kOneMin{{60}};
+  static constexpr const std::array<size_t, 1> kTenMin{{600}};
   static constexpr const std::array<size_t, 2> kOneMinTenMin{{60, 600}};
   static constexpr const std::array<size_t, 3> kOneMinTenMinHour{
       {60, 600, 3600}};