Add docs for Redis HLL counters

Problem to solve

Further details

Add docs in Telemetry guide on how to use HLL class https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/redis/hll.rb

Current implementations

• Gitlab::Analytics::UniqueVisits

• Gitlab::UsageDataCounters::TrackUniqueActions

Proposal

Who can address the issue

Other links/references