feat(buffer_worker): log expired message count

Fixes https://emqx.atlassian.net/browse/EMQX-10165

```
iex(emqx@127.0.0.1)38> 2023-06-21T11:09:35.569404-03:00 [info] msg: buffer_worker_dropped_expired_messages, mfa: emqx_resource_buffer_worker:log_expired_messge_count/1, line: 982, expired_count: 900, resource_id: <<"bridge:webhook:webhook">>, worker_index: 3
```

apps/emqx_resource/src/emqx_resource_buffer_worker.erl

@@ -927,6 +927,7 @@ merge_counters(OldCounters, DeltaCounters) ->
 flush_metrics(Data = #{id := Id, counters := Counters}) ->
     bump_counters(Id, Counters),
     set_gauges(Data),
+    log_expired_message_count(Data),
     ensure_metrics_flush_timer(Data#{counters := #{}}).
 
 -spec ensure_metrics_flush_timer(data()) -> data().
@@ -966,6 +967,22 @@ do_bump_counters1(dropped_resource_not_found, Val, Id) ->
 do_bump_counters1(dropped_resource_stopped, Val, Id) ->
     emqx_resource_metrics:dropped_resource_stopped_inc(Id, Val).
 
+-spec log_expired_message_count(data()) -> ok.
+log_expired_message_count(_Data = #{id := Id, index := Index, counters := Counters}) ->
+    ExpiredCount = maps:get(dropped_expired, Counters, 0),
+    case ExpiredCount > 0 of
+        false ->
+            ok;
+        true ->
+            ?SLOG(info, #{
+                msg => "buffer_worker_dropped_expired_messages",
+                resource_id => Id,
+                worker_index => Index,
+                expired_count => ExpiredCount
+            }),
+            ok
+    end.
+
 -spec set_gauges(data()) -> ok.
 set_gauges(_Data = #{id := Id, index := Index, queue := Q, inflight_tid := InflightTID}) ->
     emqx_resource_metrics:queuing_set(Id, Index, queue_count(Q)),

changes/ce/feat-11115.en.md

@@ -0,0 +1 @@
+Added info logs to indicate when buffered messages are dropped due to time-to-live (TTL) expiration.