Set plug-in specific labels for decision logging

[anderseknert]

The -policy-file option comes with its own decision logger which prints decisions to console. While the new -config-file option and "real" decision logging is preferable, one thing missing there is the labels set by the "fake" decision logger:

	labels := map[string]string{
		"app":            "opa-docker-authz",
		"id":             p.instanceID,
		"opa_version":    version_pkg.OPAVersion,
		"plugin_version": version_pkg.Version,
	}

It would be nice if we could have these labels provided also for the -config-file decision logger. The SDK doesn't currently provide any helpers for this, but that could change. If not we'll need to convert the config []bytes -> config.Config -> update labels -> config []byte.