Register custom metrics

Classes that implement MFR\T3PromClient\Metrics\MetricsInterface are automatically tagged by the Dependency Injection container and added to the Metrics registry, allowing the registration of custom metrics by extension developers.

1. Create a new PHP class that implements MFR\T3PromClient\Metrics\MetricsInterface
2. Implement the contracts.
3. Flush all TYPO3 caches.