Optimise creation of DM (#16819)

* Optimise creation of dm

* Handle direct channels with the same user

* Cover GetMany with specs and add it on tha cache layer as well

* Fix specs by handling user dming themselves

* Apply PR suggestions

* Apply PR suggestions

* Use require.NoError instead of require.Nil on userstore test

* Improve readability of GetOrCreateDirectChannel

* Apply PR suggestions

* Update layers

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/opentracinglayer/opentracinglayer.go

@@ -8703,6 +8703,24 @@ func (s *OpenTracingLayerUserStore) GetKnownUsers(userID string) ([]string, erro
 	return result, err
 }
 
+func (s *OpenTracingLayerUserStore) GetMany(ids []string) ([]*model.User, error) {
+	origCtx := s.Root.Store.Context()
+	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "UserStore.GetMany")
+	s.Root.Store.SetContext(newCtx)
+	defer func() {
+		s.Root.Store.SetContext(origCtx)
+	}()
+
+	defer span.Finish()
+	result, err := s.UserStore.GetMany(ids)
+	if err != nil {
+		span.LogFields(spanlog.Error(err))
+		ext.Error.Set(span, true)
+	}
+
+	return result, err
+}
+
 func (s *OpenTracingLayerUserStore) GetNewUsersForTeam(teamId string, offset int, limit int, viewRestrictions *model.ViewUsersRestrictions) ([]*model.User, error) {
 	origCtx := s.Root.Store.Context()
 	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "UserStore.GetNewUsersForTeam")