Track unique user action users_updating_work_item_labels

What does this MR do and why?

Implements service ping to track users updating work item labels.

How to set up and validate locally

You can test the counters work manually in the rails console by doing something like:

Feature.enable :track_work_items_activity

Gitlab::UsageDataCounters::WorkItemActivityUniqueCounter.track_work_item_labels_changed_action(author: User.first)

Gitlab::UsageDataCounters::HLLRedisCounter.unique_events(event_names: 'users_updating_work_item_labels', start_date: Date.current.beginning_of_week, end_date: Date.current.next_week)

Updating labels on a work item via the GraphQL API should also increment the counter. How to use that mutation is described in !100098 (merged)

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 #339756 (closed)