Support Adding Arbitrary Metric Labels when imported as a Package

[caleblloyd]

Add support for adding arbitrary metric labels to a Surveyor instance, that are in-turn added to each metric.

This is meant to support the case where Surveyor is imported as a package and configured to scrape multiple Clusters. It does not need to be exposed by the CLI, as the CLI is meant to only scrape 1 cluster, and Arbitrary Metrics Labels can be added in the scraper configuration.