Add `nginx` error grafana monitoring

### Feature Request

The idea is to have visibility into why 429s are happening (e.g. which intervention is triggering it). This would also let us monitoring max_conns.

### Breakdown

Implementation Details (for maintainers)

#### Related files

Refer to [this map of common Endpoints](https://docs.openlibrary.org/developers/backend/endpoints.html):
*

#### Requirements Checklist
Checklist of requirements that need to be satisfied in order for this issue to be closed:
* [ ]

#### Stakeholders

*

#### Instructions for Contributors

* **Before** [creating a new branch](https://docs.openlibrary.org/developers/tools/git.html#making-changes-and-creating-a-pull-request) or pushing up changes to a PR, please first [run these commands](https://docs.openlibrary.org/developers/tools/git.html#working-on-your-branch) to ensure your repository is up to date, as the pre-commit bot may add commits to your PRs upstream.

Start with the linked map of common Endpoints and identify where nginx 429 responses and max_conns are exposed. Define the Grafana monitoring needed to show why 429s occur and to monitor max_conns, then confirm both views are available and useful before closing the issue.

Written by the indexing model from the issue text.