[V1][Metrics][Plugin] Add plugin support for custom StatLoggerBase implementations (#22456)

Signed-off-by: tovam <tovam@pliops.com>

docs/design/plugin_system.md

@@ -41,7 +41,7 @@ Every plugin has three parts:
 
 1. **Plugin group**: The name of the entry point group. vLLM uses the entry point group `vllm.general_plugins` to register general plugins. This is the key of `entry_points` in the `setup.py` file. Always use `vllm.general_plugins` for vLLM's general plugins.
 2. **Plugin name**: The name of the plugin. This is the value in the dictionary of the `entry_points` dictionary. In the example above, the plugin name is `register_dummy_model`. Plugins can be filtered by their names using the `VLLM_PLUGINS` environment variable. To load only a specific plugin, set `VLLM_PLUGINS` to the plugin name.
-3. **Plugin value**: The fully qualified name of the function to register in the plugin system. In the example above, the plugin value is `vllm_add_dummy_model:register`, which refers to a function named `register` in the `vllm_add_dummy_model` module.
+3. **Plugin value**: The fully qualified name of the function or module to register in the plugin system. In the example above, the plugin value is `vllm_add_dummy_model:register`, which refers to a function named `register` in the `vllm_add_dummy_model` module.
 
 ## Types of supported plugins
 
@@ -51,6 +51,8 @@ Every plugin has three parts:
 
 - **IO Processor plugins** (with group name `vllm.io_processor_plugins`): The primary use case for these plugins is to register custom pre/post processing of the model prompt and model output for pooling models. The plugin function returns the IOProcessor's class fully qualified name.
 
+- **Stat logger plugins** (with group name `vllm.stat_logger_plugins`): The primary use case for these plugins is to register custom, out-of-the-tree loggers into vLLM. The entry point should be a class that subclasses StatLoggerBase.
+
 ## Guidelines for Writing Plugins
 
 - **Being re-entrant**: The function specified in the entry point should be re-entrant, meaning it can be called multiple times without causing issues. This is necessary because the function might be called multiple times in some processes.