Add docs for Redis HLL counters

doc/development/telemetry/usage_ping.md

@@ -213,11 +213,47 @@ Arguments:
 - `counter`: a counter from `Gitlab::UsageDataCounters`, that has `fallback_totals` method implemented
 - or a `block`: which is evaluated
 
+#### Ordinary Redis Counters
+
+Examples of implementation:
+
+- Using Redis methods [`INCR`](https://redis.io/commands/incr), [`GET`](https://redis.io/commands/get), and [`Gitlab::UsageDataCounters::WikiPageCounter`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_data_counters/wiki_page_counter.rb)
+- Using Redis methods [`HINCRBY`](https://redis.io/commands/hincrby), [`HGETALL`](https://redis.io/commands/hgetall), and [`Gitlab::UsageCounters::PodLogs`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_counters/pod_logs.rb)
+
+#### Redis HLL Counters
+
+With `Gitlab::Redis::HLL` we have available data structures used to count unique values.
+
+Implemented using Redis methods [PFADD](https://redis.io/commands/pfadd) and [PFCOUNT](https://redis.io/commands/pfcount).
+
+Recommendations:
+
+- Key should expire in 29 days.
+- If possible, data granularity should be a week. For example a key could be composed from the metric's name and week of the year, `2020-33-{metric_name}`.
+- Use a [feature flag](../../operations/feature_flags.md) in order to have a control over the impact when adding new metrics.
+- If possible, data granularity should be week, for example a key could be composed from metric name and week of the year, 2020-33-{metric_name}
+- Use a [feature flag](../../operations/feature_flags.md) in order to have a control over the impact when adding new metrics
+
+Examples of implementation:
+
+- [`Gitlab::UsageDataCounters::TrackUniqueActions`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_data_counters/track_unique_actions.rb)
+- [`Gitlab::Analytics::UniqueVisits`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/analytics/unique_visits.rb)
+
 Example of usage:
 
 ```ruby
+# Redis Counters
 redis_usage_data(Gitlab::UsageDataCounters::WikiPageCounter)
 redis_usage_data { ::Gitlab::UsageCounters::PodLogs.usage_totals[:total] }
+
+# Redis HLL counter
+counter = Gitlab::UsageDataCounters::TrackUniqueActions
+redis_usage_data do
+          counter.count_unique_events(
+            event_action: Gitlab::UsageDataCounters::TrackUniqueActions::PUSH_ACTION,
+            date_from: time_period[:created_at].first,
+            date_to: time_period[:created_at].last
+          )
 ```
 
 Note that Redis counters are in the [process of being deprecated](https://gitlab.com/gitlab-org/gitlab/-/issues/216330) and you should instead try to use Snowplow events instead. We're in the process of building [self-managed event tracking](https://gitlab.com/gitlab-org/telemetry/-/issues/373) and once this is available, we will convert all Redis counters into Snowplow events.