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

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

tests/v1/metrics/test_engine_logger_apis.py

@@ -4,33 +4,13 @@ import copy
 
 import pytest
 
+from tests.plugins.vllm_add_dummy_stat_logger.dummy_stat_logger.dummy_stat_logger import (  # noqa E501
+    DummyStatLogger,
+)
 from vllm.v1.engine.async_llm import AsyncEngineArgs, AsyncLLM
 from vllm.v1.metrics.ray_wrappers import RayPrometheusStatLogger
 
 
-class DummyStatLogger:
-    """
-    A dummy stat logger for testing purposes.
-    Implements the minimal interface expected by StatLoggerManager.
-    """
-
-    def __init__(self, vllm_config, engine_idx):
-        self.vllm_config = vllm_config
-        self.engine_idx = engine_idx
-        self.recorded = []
-        self.logged = False
-        self.engine_initialized = False
-
-    def record(self, scheduler_stats, iteration_stats, engine_idx):
-        self.recorded.append((scheduler_stats, iteration_stats, engine_idx))
-
-    def log(self):
-        self.logged = True
-
-    def log_engine_initialized(self):
-        self.engine_initialized = True
-
-
 @pytest.fixture
 def log_stats_enabled_engine_args():
     """