User guide for metric engine

[killme2008]

We currently have a contributor guide available for the metric engine at https://docs.greptime.com/contributor-guide/datanode/metric-engine. However, the crucial user guide appears to be missing.

It may include:

1. How to create and manage physical tables
2. How to create and manage logical tables
3. Other related info

[nicecui]

In what scenarios do users need to create and manage physical/logical tables?

[killme2008]

In what scenarios do users need to create and manage physical/logical tables?

It's the questions that the user guide must address.