The memory usage of greptimedb-datanode is too high！

### What type of enhancement is this?

Performance

### What does the enhancement do?

k8s集群内部部署的greptimedb，接入的是prometheus的数据，存储12天的数据，datanode内存使用量超高，最高已经达到100G以上，总会被 oom kill掉。 greptimedb的版本是 最新的 v0.15.0

<img width="1991" height="710" alt="Image" src="https://github.com/user-attachments/assets/7ce792c8-cff7-43ea-91b8-beabdb0786e7" />

### Implementation challenges

性能很差