[FE] Instance stats: Projects vs groups line chart

What does this MR do?

Adds a line chart with monthly averaged counts for:

• Projects
• Groups

Related to #246490 (closed)

Screenshots

With data	One metric fails to load	No data

Feature flag

Requires the instance_statistics feature flag

Testing

1. Enable the feature flag
2. Seed data FILTER=instance_statistics bundle exec rake db:seed_fu
3. Sign in as an admin
4. Navigate to /admin/instance_statistics

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