Add an instance variable to self monitoring dashboard

What does this MR do?

Add an instance variable to the self monitoring default dashboard. This allows users to provide a regular expression to filter the resource usage charts by the instance label. The default value for the variable is .* which should display all instances on the chart (nothing will be filtered).

This is useful for GitLab installations that have a large number of servers. Users can change the regular expression filter to only display resource usage of some instances, rather than all.

To test out this dashboard, you can use an Omnibus installation:

1. Setup Omnibus: https://docs.gitlab.com/omnibus/installation/index.html
2. Create a new custom dashboard in the self monitoring project: https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#adding-a-new-dashboard-to-your-project
3. Add the yaml from this MR into your custom dashboard.
4. Go to the metrics dashboard of the self monitoring project (Operations > Metrics), and view the dashboard that you added.

self_monitoring_dashboard_instance_filter

Issue: #212882 (closed)

Screenshots

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