Track work item iteration update

What does this MR do and why?

• Adds RedisHLL count metrics (7d/28d) for work item iteration usage
• Adds the added metric to the WI activity aggregated OR metric.

How to set up and validate locally

The added metric (and the updated aggregate metrics) are behind a feature flag that's been defaulted (:track_work_items_activity.)

To see the added event in action, directly increment the counter in the Rails console:

Gitlab::UsageDataCounters::HLLRedisCounter.track_event(:users_updating_work_item_iteration, values: 1)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #367456 (closed)