[server] add the metric of KV preWriteBuffer

### Search before asking

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

### Motivation

Under high write throughput on a single tablet server, if replication falls behind, the KV preWriterBuffer may be unable to release buffered data because the high watermark has not advanced, eventually leading to OOM on the tablet server.

for the above case, we should do the following ways to eliminate risk
1. introduce the metric to observe
2. introduce the backpressure mechanism to limit client writing speed.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start by locating the tablet-server implementation and references to the KV preWriterBuffer, then trace how buffered data and the high watermark are handled during replication lag. The issue is done when the buffer can be observed through a metric and the proposed client-write backpressure behavior is defined, implemented, and covered by relevant tests.

Written by the indexing model from the issue text.