[rollout] fix: wrong display about Prometheus when using SGLang.

[jsfanfanfan]

What does this PR do?

Fix the issue where vLLM is always displayed when using other rollout engines.

[gemini-code-assist]

Code Review

This pull request aims to fix an issue where the rollout engine type was not correctly displayed for Prometheus monitoring, always showing 'vLLM'. The changes introduce a rollout_name parameter to update_prometheus_config and use it to display the correct engine type in a log message.

While this is a good step, the fix is incomplete. The actual job_name in the generated Prometheus configuration file is still hardcoded to 'rollout'. This means that in Prometheus and Grafana, the jobs will not be distinguished by their engine type (e.g., 'vllm' vs 'sglang'), which seems to be the core goal. I've added a comment with a suggestion to make the job_name dynamic to fully resolve the issue.

[wuxibin89]

@jsfanfanfan Please format code according to: https://github.com/volcengine/verl/blob/main/CONTRIBUTING.md