feat: Retrieving a metric by the name

What

The method Labkit::Metrics::Client.get returns a predefined metric, be it a counter, gauge, summary, or histogram. Or returns nil if it does not exist.

Why

While fixing some specs in the MR, having this getter would be nice to retrieve metrics from the registry from its name since we don't have direct access to the registry in Labkit::Metrics.

This way we can control if registry is properly initialized before trying to retrieve the metric.