[FEATURE] Show read_used_buffer_size in DashBoard

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### Describe the feature

We only show `used_buffer_size` in the dashboard for now. We should show `read_used_buffer_size` too.

### Motivation

_No response_

### Describe the solution

_No response_

### Additional context

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

Start by locating the dashboard code that currently displays `used_buffer_size` and trace how that value is supplied. Add the corresponding `read_used_buffer_size` display, then verify that both values appear correctly in the dashboard.

Written by the indexing model from the issue text.