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

greptimedb/config/datanode.example.toml

Line 351 in fe1cfbf

global_write_buffer_size = "1GB"

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

greptimedb/config/datanode.example.toml

Line 432 in fe1cfbf

mem_threshold_on_create = "auto"

Implementation challenges

No response