GH-9612 Add plugin API for GetChannelStats method (#9627)

* Add GetChannelStats plugin api

* Fix to return channel stats correctly

* Add server version; Handle error idiomatically

app/plugin_api.go

@@ -261,6 +261,14 @@ func (api *PluginAPI) GetChannelsForTeamForUser(teamId, userId string, includeDe
 	return api.app.GetChannelsForUser(teamId, userId, includeDeleted)
 }
 
+func (api *PluginAPI) GetChannelStats(channelId string) (*model.ChannelStats, *model.AppError) {
+	memberCount, err := api.app.GetChannelMemberCount(channelId)
+	if err != nil {
+		return nil, err
+	}
+	return &model.ChannelStats{ChannelId: channelId, MemberCount: memberCount}, nil
+}
+
 func (api *PluginAPI) GetDirectChannel(userId1, userId2 string) (*model.Channel, *model.AppError) {
 	return api.app.GetDirectChannel(userId1, userId2)
 }