Add ACL Authentication and Authorization Metrics to Nomad

### Proposal

Hello,

While configuring alerts for my Nomad cluster, I noticed that there are no existing metrics to track ACL-related events. Specifically, I am interested in monitoring denied RPC requests due to insufficient permissions.

It would be very useful to have built-in metrics that provide insights into authentication and authorization failures, including failed RPC requests due to ACL violations. This would enable more proactive alerting without relying on log scanning, which currently requires setting the log level to **debug** to capture failed authentication attempts—a less-than-ideal solution due to the increased verbosity.

### Use-cases

The primary use case is to enhance security monitoring and infrastructure management by making it easier to detect and respond to failed authentication or authorization attempts. This could help identify potential security breaches or misconfigurations in a more efficient manner than parsing debug logs.

Start by tracing Nomad’s ACL authentication and authorization failure paths and reviewing how existing metrics and log events are recorded. Determine the metric scope and failure categories needed for denied RPC requests, then verify that the resulting metrics support security alerting without requiring debug-level logs.

Written by the indexing model from the issue text.