Migrate MergeRequestWidgetExtensionCounter to use a Metric Instrumentation class

As a part of #339445 (closed), we want to move the MergeRequestWidgetExtensionCounter from UsageDataCounters to use basic RedisMetric class, defined with metrics yaml file parameters. This will require moving this class from COUNTERS to COUNTERS_MIGRATED_TO_INSTRUMENTATION_CLASSES const in the UsageDataCounters class.