More realistic instance statistics seed file

What does this MR do?

This MR generates more realistic instance statistics dummy data. Since delete events are rather rare in GitLab, we assume that daily record counts will slowly increase. In some cases (5% chance) we'll decrease the previous count.

How to test it:

rails c

Analytics::InstanceStatistics::Measurement.delete_all

Run: FILTER=instance_statistics bundle exec rake db:seed_fu

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