[lake/tiering] Add More Tiering Monitoring Metrics

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

Introduce a set of monitoring metrics focused on the tiering process, including file/record sizes, read/write throughput.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!

The issue points to the lake/tiering area but names no files, tests, or entry points. Start by locating the tiering implementation and existing monitoring metrics, then determine the relevant file and record sizes plus read/write throughput to cover. Done means the requested tiering metrics are available and validated by the project's appropriate tests.

Written by the indexing model from the issue text.