Provide Example Usage in pmm-admin help output when user enters wrong format for custom labels

We should have our CLI tool provide enough information about the issue, if a user is adding a wrong input format, we should be able to tell them with an example what is the right way to add it. 

For example: 

I try to run: 

The output we get is:

But it would be great if move one step more towards ease of use and add another line to this output:

Purpose: Less need to find documentation, less frustration for the user.