Add monthly metric for unique user logins

What does this MR do and why?

• Originally added here: !96321 (merged)
• Updated here to indicate that this is the "SMAU" metric for Manage: !97326 (merged)
• We were then infromed via Slack that it was added as an all-time counter instead of a 28-day counter.
• "We only use 28-day counters for xMAU metrics". This MR updates the metric to be for 28d (month) rather than all time.
• Docs on time frames for metric: https://docs.gitlab.com/ee/development/service_ping/metrics_dictionary.html#metric-time_frame
• Rather than removing the existing metric, which requires this process, we are copying the existing metric to be a monthly metric as sell. So now we have an all-time metric and a monthly metric.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.