FR: Improved Metrics Collection

Running vault in kubernetes as a cronjob, collecting metrics is failing because Vault Benchmark shuts down immediately after printing results we’re not able to scrape the result metrics quick enough. This is running on port 2112 at the moment.

Metrics are output from the built-in HTTP load testing tool - [Vegeta](https://github.com/tsenart/vegeta?tab=readme-ov-file#prometheus-support)

Ideally we'd want it to get Vault Benchmark running in an automated way (K8s Cronjob), once a day to load test for 15 minutes in our development environment to confirm everything is looking good, we only get alerted if the results are outside expected thresholds. To do this we would need metrics to be continuously outputting the metrics or stick around for a bit at the end of the test so we can scrape the metrics.
Thanks