MB-56094: Dump num-items for all checkpints in CM

Currently we report num_items only for the open checkpoint.
Useful information in general, particularly useful when inspecting
stats for checkpoints that have been touched by ItemExpel.

Note: The CM level 'num_open_checkpoint_items' is now reduntant but
used by some ep_testsuite test. I'm removing that in a dedicated patch.

Change-Id: Ic4401c9bb65eb26c870df7e6d949e6ab9cd1d728
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/189987
Reviewed-by: Dave Rigby <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: paolococchi

engines/ep/docs/stats.org

@@ -1005,6 +1005,7 @@ each stat name.
 | num_checkpoint_items                | Number of total items in a checkpoint     |
 |                                     | datastructure                             |
 | num_open_checkpoint_items           | Number of items in the open checkpoint    |
+|                                     | (empty item excluded)                     |
 | num_checkpoints                     | Number of all checkpoints in the bucket,  |
 |                                     | including all Vbuckets/CMs/Destroyers     |
 | num_checkpoints_pending_destruction | Number of checkpoints detached from CMs   |
@@ -1034,6 +1035,8 @@ prefixed with the vbucket and the id of the Checkpoint, e.g. "vb_0:id_52:state"
 | mem_usage_key_index_overhead     | Bytes consumed by the key index. Accounts |
 |                                  | both struct internals and keys. Computed  |
 |                                  | by checkpoint counters                    |
+| num_items                        | Number of items queued in the checkpoint  |
+|                                  | (empty item excluded)                     |
 
 ** Memory Stats
 

engines/ep/src/checkpoint.cc

@@ -654,6 +654,13 @@ void Checkpoint::addStats(const AddStatFn& add_stat, CookieIface& cookie) {
                      getId());
     add_casted_stat(
             buf.data(), isCheckpointHistorical(historical), add_stat, cookie);
+
+    checked_snprintf(buf.data(),
+                     buf.size(),
+                     "vb_%d:id_%" PRIu64 ":num_items",
+                     vbucketId.get(),
+                     getId());
+    add_casted_stat(buf.data(), getNumItems(), add_stat, cookie);
 }
 
 void Checkpoint::detachFromManager() {

engines/ep/tests/ep_testsuite.cc

@@ -6385,7 +6385,8 @@ static enum test_result test_mb19687_fixed(EngineIface* h) {
               "vb_0:id_1:type",
               "vb_0:id_1:historical",
               "vb_0:id_1:visible_snap_end",
-              "vb_0:id_1:highest_expelled_seqno"}},
+              "vb_0:id_1:highest_expelled_seqno",
+              "vb_0:id_1:num_items"}},
             {"checkpoint 0",
              {"vb_0:mem_usage",
               "vb_0:mem_usage_queued_items",
@@ -6408,7 +6409,8 @@ static enum test_result test_mb19687_fixed(EngineIface* h) {
               "vb_0:id_1:type",
               "vb_0:id_1:historical",
               "vb_0:id_1:visible_snap_end",
-              "vb_0:id_1:highest_expelled_seqno"}},
+              "vb_0:id_1:highest_expelled_seqno",
+              "vb_0:id_1:num_items"}},
             {"uuid", {"uuid"}},
             {"kvstore",
              {"rw_0:backend_type",