Add ability to prefix metrics with string

[andheiberg-gelato]

I believe it's a fairly common use-case to prefix your services metrics with a identifiable string instead of relying fully on labels. The main benefit I'm told is the ease of discovering what metrics are available for a service.

The extension is a fairly convenient place to do this https://github.com/tweedegolf/prometheus-bundle/blob/master/src/DependencyInjection/TweedeGolfPrometheusExtension.php#L39

Else https://github.com/tweedegolf/prometheus-client/blob/master/src/Validator.php#L72