[CI]Add regression tests to ensure the async engine generates metrics (#4524)

tests/metrics/test_metrics.py

 import pytest
+from prometheus_client import REGISTRY
+
+from vllm import EngineArgs, LLMEngine
+from vllm.engine.arg_utils import AsyncEngineArgs
+from vllm.engine.async_llm_engine import AsyncLLMEngine
+from vllm.sampling_params import SamplingParams
 
 MODELS = [
     "facebook/opt-125m",
@@ -68,3 +74,91 @@ def test_metric_counter_generation_tokens(
     assert vllm_generation_count == metric_count, (
         f"generation token count: {vllm_generation_count!r}\n"
         f"metric: {metric_count!r}")
+
+
+@pytest.mark.parametrize("model", MODELS)
+@pytest.mark.parametrize("dtype", ["half"])
+@pytest.mark.parametrize("max_tokens", [4])
+@pytest.mark.parametrize("disable_log_stats", [True, False])
+@pytest.mark.asyncio
+async def test_async_engine_log_metrics_regression(
+    example_prompts,
+    model: str,
+    dtype: str,
+    max_tokens: int,
+    disable_log_stats: bool,
+) -> None:
+    """
+    Regression test ensuring async engine generates metrics
+    when disable_log_stats=False
+    (see: https://github.com/vllm-project/vllm/pull/4150#pullrequestreview-2008176678)
+    """
+    engine_args = AsyncEngineArgs(model=model,
+                                  dtype=dtype,
+                                  disable_log_stats=disable_log_stats)
+    async_engine = AsyncLLMEngine.from_engine_args(engine_args)
+    for i, prompt in enumerate(example_prompts):
+        results = async_engine.generate(
+            prompt,
+            SamplingParams(max_tokens=max_tokens),
+            f"request-id-{i}",
+        )
+        # Exhaust the async iterator to make the async engine work
+        async for _ in results:
+            pass
+
+    assert_metrics(async_engine.engine, disable_log_stats,
+                   len(example_prompts))
+
+
+@pytest.mark.parametrize("model", MODELS)
+@pytest.mark.parametrize("dtype", ["half"])
+@pytest.mark.parametrize("max_tokens", [4])
+@pytest.mark.parametrize("disable_log_stats", [True, False])
+def test_engine_log_metrics_regression(
+    example_prompts,
+    model: str,
+    dtype: str,
+    max_tokens: int,
+    disable_log_stats: bool,
+) -> None:
+    engine_args = EngineArgs(model=model,
+                             dtype=dtype,
+                             disable_log_stats=disable_log_stats)
+    engine = LLMEngine.from_engine_args(engine_args)
+    for i, prompt in enumerate(example_prompts):
+        engine.add_request(
+            f"request-id-{i}",
+            prompt,
+            SamplingParams(max_tokens=max_tokens),
+        )
+    while engine.has_unfinished_requests():
+        engine.step()
+
+    assert_metrics(engine, disable_log_stats, len(example_prompts))
+
+
+def assert_metrics(engine: LLMEngine, disable_log_stats: bool,
+                   num_requests: int) -> None:
+    if disable_log_stats:
+        with pytest.raises(AttributeError):
+            _ = engine.stat_logger
+    else:
+        assert (engine.stat_logger
+                is not None), "engine.stat_logger should be set"
+        # Ensure the count bucket of request-level histogram metrics matches
+        # the number of requests as a simple sanity check to ensure metrics are
+        # generated
+        labels = {'model_name': engine.model_config.model}
+        request_histogram_metrics = [
+            "vllm:e2e_request_latency_seconds",
+            "vllm:request_prompt_tokens",
+            "vllm:request_generation_tokens",
+            "vllm:request_params_best_of",
+            "vllm:request_params_n",
+        ]
+        for metric_name in request_histogram_metrics:
+            metric_value = REGISTRY.get_sample_value(f"{metric_name}_count",
+                                                     labels)
+            assert (
+                metric_value == num_requests), "Metrics should be collected"