Usage activity by stage for past month

What does this MR do?

Adds a version of usage_activity_by_stage called usage_activity_by_stage_monthly

• 1 month is counted as 28 days backwards by created_at day created_at: 28.days.ago..DateTime.current }
• The tests simply create a "60 day ago" version of the data and expect the usage_activity_by_stage_monthly to be half of the usage_activity_by_stage

Issue #201764 (closed)

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.

Reviewing

On rails console you could create them as:

time_period = { created_at: 28.days.ago..DateTime.current }
::Gitlab::UsageData.usage_activity_by_stage(:usage_activity_by_stage_monthly, time_period)
::Gitlab::UsageData.usage_activity_by_stage

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team