Make cmdlet metadata available in non-debug build

You can find the metadata of a cmdlet (for example "subject") as comments in the generated cmdlet code. However it is only available in debug mode (`autorest --debug`).

Considering (a) this information is quite useful for writing directives and (b) it is comment so it won't increase assembly size, **I suggest making cmdlet metadata also available in non-debug build.**

At last, add a guide in [directives.md](https://github.com/Azure/autorest.powershell/blob/main/docs/directives.md) to describe how to find "subject" of a cmdlet.