specify labels unique to a process type

[kevin-cantwell]

Use env vars that have a pattern like BACKPLANE_LABELS_<proc> to specify unique labels for a process type.

When using multiple process types as web servers, one typically would wish to route traffic differently between them. Since the config is shared across all process types, BACKPLANE_LABELS is insufficient to achieve this.

Eg:

backplane route www.mydomain.com "endpoint=www.mydomain.com"
backplane route admin.mydomain.com "endpoint=admin.mydomain.com"

Procfile:

web: web
admin: admin
worker: worker

BACKPLANE_LABELS_web="endpoint=www.mydomain.com"
BACKPLANE_LABELS_admin="endpoint=admin.mydomain.com"