Supports auto as values for all numeric configurations with default settings

[killme2008]

What type of enhancement is this?

Configuration

What does the enhancement do?

The numeric configurations like

https://github.com/GreptimeTeam/greptimedb/blob/fe1cfbf2b3984eaaefd688e056ce5824139d4cf2/config/datanode.example.toml#L351

It has a default value that depends on system memory, but we hard-coded it in the sample configuration, it's not expected.

I create a PR #4495 to comment out all of them. But a better way is to make them support auto as the default value just like

https://github.com/GreptimeTeam/greptimedb/blob/fe1cfbf2b3984eaaefd688e056ce5824139d4cf2/config/datanode.example.toml#L432

Implementation challenges

No response