[Metrics] Provide a metric for consumer lag

### Search before asking

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

### Motivation

Currently, Fluss provides `currentFetchEventTimeLag`, `currentEmitEventTimeLag`, and `pendingRecords` metrics in the Flink source.

However, it doesn't support out of the box a metric for `consumer offset` (similar to the `consumer lag` in Kafka), which can be useful to users.

For reference: https://www.redpanda.com/guides/kafka-performance-kafka-consumer-lag

### Solution

We should support this metric in Fluss so that all the clients/connectors can leverage it.

### Anything else?

_No response_

### Willingness to contribute

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