[V1][Metrics][Plugin] Add plugin support for custom StatLoggerBase implementations via vllm.stat_logger_plugins

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

Author: ptovam

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 poling 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.

vllm/v1/metrics/loggers.py

@@ -12,6 +12,7 @@
 from vllm.distributed.kv_transfer.kv_connector.v1.metrics import (
     KVConnectorLogging)
 from vllm.logger import init_logger
+from vllm.plugins import load_plugins_by_group
 from vllm.v1.core.kv_cache_utils import PrefixCachingMetrics
 from vllm.v1.engine import FinishReason
 from vllm.v1.metrics.prometheus import unregister_vllm_metrics
@@ -657,6 +658,23 @@ def build_1_2_5_buckets(max_value: int) -> list[int]:
     return build_buckets([1, 2, 5], max_value)
 
 
+def load_stat_logger_plugin_factories() -> list[StatLoggerFactory]:
+    factories: list[StatLoggerFactory] = []
+
+    for name, plugin_class in load_plugins_by_group(
+            "vllm.stat_logger_plugins").items():
+        if not isinstance(plugin_class, type) or not issubclass(
+                plugin_class, StatLoggerBase):
+            logger.warning(
+                "Stat logger plugin %s is not a valid subclass "
+                "of StatLoggerBase. Skipping.", name)
+            continue
+
+        factories.append(plugin_class)
+
+    return factories
+
+
 class StatLoggerManager:
     """
     StatLoggerManager:
@@ -684,6 +702,9 @@ def __init__(
         if custom_stat_loggers is not None:
             factories.extend(custom_stat_loggers)
 
+        # Load plugin-based stat loggers
+        factories.extend(load_stat_logger_plugin_factories())
+
         if enable_default_loggers and logger.isEnabledFor(logging.INFO):
             if client_count > 1:
                 logger.warning(