[DOC]Create documentation for _stats API

**What do you want to do?**
 
- [ ] Request a change to existing documentation
- [ x] Add new documentation
- [ ] Report a technical problem with the documentation
- [ ] Other 

```
 GET /_stats
GET /_stats/<metric>
```

**What other resources are available?** Provide links to related issues, POCs, steps for testing, etc.

