Add a separate metrics definition generator for Redis HLL counters

What does this MR do?

Part of #322733 (closed).

This adds a new generator gitlab:usage_metric_definition:redis_hll that creates two definitions with monthly and weekly timeframes (by running gitlab:usage_metric_definition twice) in accordance with the existing guidelines for Redis HLL metric definitions.

Skipping validation in Gitlab::Usage::MetricDefinition.definitions was needed because metrics that are generated with gitlab:usage_metric_definition are invalid at first (they require filling out some required keys like description and tier), and when the generator runs again to create a second file, Gitlab::Usage::MetricDefinition.definitions call tries to validate all metrics, including the new temporarily invalid one, and raises an exception.

Gitlab::UsageMetricDefinitionGenerator is called explicitly through the Thor API instead of using the generate command due to issues with RSpec constant stubbing. This was probably caused by generate calling the generator with a system call, spawning a new process that reloads the class, circumventing stubbing in the process. This also shows explicit relation (or coupling) to the generator we call.

Does this MR meet the acceptance criteria?

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Related to #322733 (closed)