Round average coverage to 2 decimal places

This feature is currently behind the group_coverage_data_report_graph feature flag

What does this MR do?

This MR fixes our average_coverage fields to round it up at 2 decimal places.

Why are we doing this?

Today, we are averaging all coverage data and aggregating it on our group repository analytic page.

In order to display the data nicely on the frontend we want to round it at 2 decimals.

Below a screenshot of how it currently looks like:

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