db/hints: add a metric for counting processed files

Adds a field to `end_point_hints_manager::sender`:
`_total_replayed_segments_count` which keeps track of how many segments
were replayed so far. This metric will be used to calculate the
sequence number of the last current hint segments in the queue - so
that we can implement waiting for current segments to be replayed.

db/hints/manager.cc

@@ -885,6 +885,7 @@ void manager::end_point_hints_manager::sender::send_hints_maybe() noexcept {
             }
             _segments_to_replay.pop_front();
             ++replayed_segments_count;
+            ++_total_replayed_segments_count;
         }
 
     // Ignore exceptions, we will retry sending this file from where we left off the next time.

db/hints/manager.hh

@@ -153,6 +153,7 @@ public:
             seastar::scheduling_group _hints_cpu_sched_group;
             gms::gossiper& _gossiper;
             seastar::shared_mutex& _file_update_mutex;
+            uint64_t _total_replayed_segments_count = 0;
 
         public:
             sender(end_point_hints_manager& parent, service::storage_proxy& local_storage_proxy, database& local_db, gms::gossiper& local_gossiper) noexcept;