maintenance_mode on high CPU usage

## Summary

I use CouchDB rpm packages (no docker, no k8s) on the AWS cloud with an Application load-balancer (ALB).
On ALB configured health check for `/_up` endpoint.
When the CouchDB daemon fails and starts consuming CPU I want to inform ALG about node failure.
Could you add a config param like `maintenance_mode_on_cpu_load` with an integer value (for example 60% per CPU core)? If the CPU core load rise configured value for one of the CPU cores, then enable `maintenance_mode` and return `404` error for `/_up` endpoint.

## Additional context

This allows detect CouchDB node failure on ALB side and reroute traffic to other nodes in the cluster.

Start with CouchDB’s `/_up` health-check endpoint and the existing `maintenance_mode` behavior, then trace how configuration and per-core CPU load are handled. Done means a configured CPU threshold can activate maintenance mode and make `/_up` return 404 under the requested condition, with tests covering the behavior.

Written by the indexing model from the issue text.