Add counter for checkpoint promotions that are not executed

Due to a current promotion or snapshot being taken.

Author: kjnilsson

src/ra.hrl

@@ -276,6 +276,8 @@
           "Number of checkpoint bytes written"},
          {checkpoints_promoted, ?C_RA_LOG_CHECKPOINTS_PROMOTED, counter,
           "Number of checkpoints promoted to snapshots"},
+         {checkpoints_promotions_dropped, ?C_RA_LOG_CHECKPOINTS_PROMOTIONS_DROPPED,
+          counter, "Number of checkpoints promotions that were not exectued"},
          {reserved_1, ?C_RA_LOG_RESERVED, counter, "Reserved counter"}
          ]).
 -define(C_RA_LOG_WRITE_OPS, 1).
@@ -293,6 +295,7 @@
 -define(C_RA_LOG_CHECKPOINTS_WRITTEN, 13).
 -define(C_RA_LOG_CHECKPOINT_BYTES_WRITTEN, 14).
 -define(C_RA_LOG_CHECKPOINTS_PROMOTED, 15).
+-define(C_RA_LOG_CHECKPOINTS_PROMOTIONS_DROPPED, 15).
 -define(C_RA_LOG_RESERVED, 16).
 
 -define(C_RA_SRV_AER_RECEIVED_FOLLOWER, ?C_RA_LOG_RESERVED + 1).

src/ra_log.erl

@@ -895,6 +895,7 @@ promote_checkpoint(Idx, #?MODULE{cfg = Cfg,
                                  snapshot_state = SnapState0} = State) ->
     case ra_snapshot:pending(SnapState0) of
         {_WriterPid, _IdxTerm, snapshot} ->
+            ok = incr_counter(Cfg, ?C_RA_LOG_CHECKPOINTS_PROMOTIONS_DROPPED, 1),
             %% If we're currently writing a snapshot, skip promoting a
             %% checkpoint.
             {State, []};