Add redisHLL unique values reading for metrics

config/metrics/schema/redis_hll.json

.../lib/ee/gitlab/usage/metrics/aggregates/aggregate_spec.rb

lib/gitlab/usage/metrics/aggregates/aggregate.rb

...e/metrics/aggregates/sources/calculations/intersection.rb

lib/gitlab/usage/metrics/aggregates/sources/postgres_hll.rb

lib/gitlab/usage/metrics/aggregates/sources/redis_hll.rb

...itlab/usage/metrics/instrumentations/aggregated_metric.rb

...tlab/usage/metrics/instrumentations/total_count_metric.rb

lib/gitlab/usage/time_series_storable.rb

lib/gitlab/usage_data_counters/hll_redis_counter.rb

spec/lib/gitlab/usage/metrics/aggregates/aggregate_spec.rb

...rics/aggregates/sources/calculations/intersection_spec.rb

...lab/usage/metrics/aggregates/sources/postgres_hll_spec.rb

...gitlab/usage/metrics/aggregates/sources/redis_hll_spec.rb

.../usage/metrics/instrumentations/aggregated_metric_spec.rb

spec/lib/gitlab/usage/time_series_storable_spec.rb

.../lib/gitlab/usage_data_counters/hll_redis_counter_spec.rb