[Frontend] enable custom logging for the uvicorn server (OpenAI API server)

[fpaupier]

This PR introduce the possibility to define a custom logging configuration for the uvicorn server used to expose the OpenAI API compliant API.
Previously it was possible to modify the log format of the vLLM engine but specifying a config file had no effect over the uvicorn API.
Hence you always saw something like this uvicorn log format when starting your app (log below)

INFO:     Started server process [128887]
INFO:     Waiting for application startup.
INFO:     Application startup complete.

This is limitating for use in production systems that have auditing requirement and must expose logs in a specific format to ensure compliance or just to make it easier to parse logs from various systems.

This PR extend the custom logging possibility defined in https://docs.vllm.ai/en/latest/getting_started/examples/logging_configuration.html to enable custom uvicorn logging format.

FIX #18210

---

You can test it yourself with the provided logger config below and making sure to expose it to the vLLM process by doing
export VLLM_LOGGING_CONFIG_PATH=./logger-config.json vllm serve....

{
  "version": 1,
  "disable_existing_loggers": false,
  "formatters": {
    "plain": {
      "class": "logging.Formatter",
      "format": "%(asctime)s [%(levelname)s] %(filename)s:%(lineno)d - %(funcName)s() - %(message)s",
      "datefmt": "%d-%m-%Y %H:%M:%S"
    },
    "default": {
      "()": "uvicorn.logging.DefaultFormatter",
      "format": "%(asctime)s [%(levelname)s] %(filename)s:%(lineno)d - %(funcName)s() - %(message)s",
      "datefmt": "%d-%m-%Y %H:%M:%S"
    },
    "access": {
      "()": "uvicorn.logging.AccessFormatter",
      "format": "%(asctime)s [%(levelname)s] %(client_addr)s - \"%(request_line)s\" %(status_code)s",
      "datefmt": "%d-%m-%Y %H:%M:%S"
    }
  },
  "handlers": {
    "console": {
      "class": "logging.StreamHandler",
      "formatter": "plain",
      "level": "INFO",
      "stream": "ext://sys.stdout"
    },
    "default": {
      "class": "logging.StreamHandler",
      "formatter": "default",
      "stream": "ext://sys.stderr"
    },
    "access": {
      "class": "logging.StreamHandler",
      "formatter": "access",
      "stream": "ext://sys.stdout"
    }
  },
  "loggers": {
    "vllm": {
      "handlers": ["console"],
      "level": "INFO",
      "propagate": false
    },
    "vllm.entrypoints.api_server": {
      "handlers": ["console"],
      "level": "INFO",
      "propagate": false
    },
    "uvicorn": {
      "handlers": ["default"],
      "level": "INFO",
      "propagate": false
    },
    "uvicorn.error": {
      "level": "INFO"
    },
    "uvicorn.access": {
      "handlers": ["access"],
      "level": "INFO",
      "propagate": false
    }
  }
}

You will see the following log at service startup for the uvicorn server:

20-05-2025 10:11:23 [INFO] server.py:83 - _serve() - Started server process [129561]
20-05-2025 10:11:23 [INFO] on.py:48 - startup() - Waiting for application startup.
20-05-2025 10:11:23 [INFO] on.py:62 - startup() - Application startup complete.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[fpaupier]

Hello @russellb, let me know if you need additional context on this PR or if you have remarks to address to make it green for merge

[fpaupier]

Let me know @DarkLight1337 if I can add anything to ease the review to make sure this does not diverge from main and can be merged soon.

[fpaupier]

hi @DarkLight1337 - I re-ran the CI and resolved the conflict with main to have a green PR.
→ Could we proceed with a merge or a review to limit diverging from main?
It's a small PR and very useful for our prod systems. I'd be happy to address any concerns you have with this PR

[DarkLight1337]

Move this to a separate function so we can inline the log_config argument into the existing code at L1336

[DarkLight1337]

Unnecessary change

[fpaupier]

Fixed 👌

[DarkLight1337]

LGTM, sorry for the delay!