Time period for usage activity by stage in usage data (Step 1)

Parent Issue : https://gitlab.com/gitlab-org/telemetry/issues/308

Problem

Usage ping counters only count unique users over all-time.

Proposal

Enhance the counters to count unique users over a rolling 28 day period for the counts in usage_activity_by_stage. This is needed to calculate AMAU.

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Outcome

Calculate AMAU & SMAU

Next steps (if any)