Add new Nginx 500 percentage metric

What does this MR do?

As discussed in #118788 (comment 313499557), we need to create a new metric so we can add an automatic alert for Nginx 5xx error percentages.

This adds the metric + the migration to add it, as well as updates the docs.

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

To test this, you can checkout the Local Promtheus Metrics app & follow the instructions. Create some 500 requests by visiting http://localhost:8080/500 and head back to your Metrics page to see the graph in action.

Part of #118788 (closed)