Ftr: add comment for cluster

Merge branch 'featue/addCommentForCluster' of https://github.com/watermelo/dubbo-go into featue/addCommentForCluster

updated the dubbo-go-ext image

Improve code coverage of zookeeper config center

Rft: modify registry panic msg

Feat: add comments for config

Merge branch 'featue/addCommentForConfig' of https://github.com/watermelo/dubbo-go into featue/addCommentForConfig

Mod: update the comments in metrics

Mod: modify file name metedata to metadata

Develop

Add comments for config_center

Add attribute method into Invocation and RpcInvocation

Opt: opt err handling

Add comments for common directory

Merge branch 'featue/addCommentForCommon' of https://github.com/watermelo/dubbo-go into featue/addCommentForCommon

remove lock from url