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### _Why are the changes needed?_
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Add operation and backend service metrics doc.
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [x] Add screenshots for manual tests if appropriate
- [ ] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#1607 from zhenjiaguo/add_be_metric_doc.
Closes#1607
e10bf146 [zhenjiaguo] Comment
0062f3d8 [zhenjiaguo] Metric format adjustment
af84534c [zhenjiaguo] Add metrics version compatibility description
c6f9eb26 [zhenjiaguo] Add operation and backend service method metric info doc
Authored-by: zhenjiaguo <zhenjia_guo@163.com>
Signed-off-by: Kent Yao <yao@apache.org>
3ffb12de24
