Samplerate could be dynamically adjusted or make the num samples also usable.

## Enhancement
After https://github.com/pingcap/tidb/pull/37193 and https://github.com/pingcap/tidb/pull/35232, the analyze statement has used max uint64 to retrieve data, thus avoiding blocking GC. However, this change has also caused some issues. The calculation of samplerate is based on the start of analyze. If the application keeps writing data during the analyze process, the amount of data in the table may increase significantly. As a result, the initial samplerate becomes too large, leading to excessive sampling and causing memory/CPU pressure in TiDB.

It is best to have a feedback mechanism that can dynamically adjust the default samplerate as the data volume increases, or make the "num samples" option available. Currently, it is not recommended for use according to the documentation.