feature: Provide custom MetricTags during runtime

[pboos]

Until now one could only provide additional metric tags through the configuration file application.properties. Sometimes they should be provided at runtime.

This change allows providing MetricTags at runtime for each violation.

An example of such a provider (taken from the README.md):

@Component
public class MetricTagProviderExample implements MetricTagProvider {
  @Override
  public List<MetricTag> getTagsForViolation(OpenApiViolation violation) {
    return List.of(new MetricTag("rule", violation.getRule()));
  }
}

[eleonoraborghi]

Looks good!