Rethink the standardization of label value for metrics migrated from RedisHLL to Snowplow

In https://gitlab.com/gitlab-org/gitlab/-/issues/367240, we chose a standardized format for events migrated from Redis to Snowplow.

However, as explained by @mikolaj_wawrzyniak in !94369 (comment 1056817687), the approach that we went with might not apply so well to aggregate metrics [an atomic event can belong to many different aggregates, so it could potentially have many different labels].

Implementation Plan

Context

1. Create new context schema in iglu repository https://gitlab.com/gitlab-org/iglu/-/merge_requests/79
2. Include new context in pilot set of events
3. Validate new context in pilot events with Data Team
4. Update documentation
5. Include new context into Redis HLL migrated events
6. Include new context into other Service Ping migrated metrics