There is no ability to specify custom labels during the node addition/configuration

Impact on the user: 

The user is unable to set custom label for Node so they can create their own alerts and dashboards 

STR:

• Install pmm2-client 
• configure it with a command like: 
  pmm-admin config --region=DO-1 --server-insecure-tls --server-url=https://xxx:xx@1.1.1.2:443 --az=zone1 --custom-labels="app=wp1"  --environment=Prod

Expected result 

• Node added with labels region=DO-1, az=zone1, environment=Prod, app=wp1

given result

• environment and --custom-labels flags are not accepted by the pmm-admin  

see: https://forums.percona.com/discussion/56320/question-about-pmm-setup-and-prometheus-labels 

Suggested implementation:

• *add * --custom-labels flag to pmm-admin config and pmm-agent setup commands.

How to test:

• Run pmm-agent config with --custom-labels flag
• Check that node contains custom-labels