perf: un-deprecate mongodb topology pool_size and set it to 10 by default

Benchmarks proved that it effects performance:
10 ecpool workers / 10 topology pool workers outperform
100 ecpool / 1 topology worker.

Fixes: EMQX-11825

apps/emqx_mongodb/src/emqx_mongodb.erl

@@ -131,8 +131,8 @@ fields(topology) ->
             hoconsc:mk(
                 pos_integer(),
                 #{
-                    deprecated => {since, "5.1.1"},
-                    importance => ?IMPORTANCE_HIDDEN
+                    importance => ?IMPORTANCE_HIDDEN,
+                    default => 10
                 }
             )},
         {max_overflow, fun max_overflow/1},
@@ -201,23 +201,7 @@ on_start(
             false ->
                 [{ssl, false}]
         end,
-    Topology0 = maps:get(topology, NConfig, #{}),
-    %% we fix this at 1 because we already have ecpool
-    case maps:get(pool_size, Topology0, 1) =:= 1 of
-        true ->
-            ok;
-        false ->
-            ?SLOG(
-                info,
-                #{
-                    msg => "mongodb_overriding_topology_pool_size",
-                    connector => InstId,
-                    reason => "this option is deprecated; please set `pool_size' for the connector",
-                    value => 1
-                }
-            )
-    end,
-    Topology = Topology0#{pool_size => 1},
+    Topology = maps:get(topology, NConfig, #{}),
     Opts = [
         {mongo_type, init_type(NConfig)},
         {hosts, Hosts},

changes/ee/perf-12497.en.md

@@ -0,0 +1,3 @@
+Improve MongoDB connector performance.
+
+- [mongodb-erlang PR](https://github.com/emqx/mongodb-erlang/pull/41)