[V1][Metrics] Support vllm:cache_config_info

prometheus_client has support for Info metrics which are equivalent to
a Gauge whose value is permanently set to 1, but exposes interesting
key/value pair information via labels. This is used for information
about an instance that does not change - so it only needs to be
observed at startup - and allows comparing across instances in
Prometheus.

We use this concept for the vllm:cache_config_info metric:

```
vllm:cache_config_info{block_size="16",cache_dtype="auto",calculate_kv_scales="False",cpu_offload_gb="0",enable_prefix_caching="False",gpu_memory_utilization="0.9",...} 1.0
```

However, prometheus_client has never supported Info metrics in
multiprocessing mode - for unclear reasons. We simply use a Gauge
metric set to 1 and multiprocess_mode="mostrecent" instead.

Signed-off-by: Mark McLoughlin <[email protected]>

Author: markmc

tests/entrypoints/openai/test_metrics.py

@@ -230,6 +230,7 @@ async def test_metrics_counts(server: RemoteOpenAIServer,
     "vllm:prompt_tokens_total",
     "vllm:generation_tokens_total",
     "vllm:iteration_tokens_total",
+    "vllm:cache_config_info",
     "vllm:request_success_total",
     "vllm:request_prompt_tokens_sum",
     "vllm:request_prompt_tokens_bucket",

vllm/config.py

@@ -84,6 +84,12 @@ def compute_hash(self) -> str:
         ...
 
 
+class SupportsMetricsInfo(Protocol):
+
+    def metrics_info(self) -> Dict[str, str]:
+        ...
+
+
 class ModelImpl(str, enum.Enum):
     AUTO = "auto"
     VLLM = "vllm"

vllm/engine/metrics.py

@@ -8,9 +8,8 @@
 import numpy as np
 import prometheus_client
 
-from vllm.config import VllmConfig
-from vllm.engine.metrics_types import (StatLoggerBase, Stats,
-                                       SupportsMetricsInfo)
+from vllm.config import SupportsMetricsInfo, VllmConfig
+from vllm.engine.metrics_types import StatLoggerBase, Stats
 from vllm.executor.ray_utils import ray
 from vllm.logger import init_logger
 

vllm/engine/metrics_types.py

@@ -15,9 +15,9 @@
 import time
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
-from typing import Dict, List, Optional, Protocol
+from typing import List, Optional
 
-from vllm.config import VllmConfig
+from vllm.config import SupportsMetricsInfo, VllmConfig
 from vllm.spec_decode.metrics import SpecDecodeWorkerMetrics
 
 
@@ -70,12 +70,6 @@ class Stats:
     spec_decode_metrics: Optional["SpecDecodeWorkerMetrics"] = None
 
 
-class SupportsMetricsInfo(Protocol):
-
-    def metrics_info(self) -> Dict[str, str]:
-        ...
-
-
 class StatLoggerBase(ABC):
     """Base class for StatLogger."""
 

vllm/v1/metrics/loggers.py

@@ -7,7 +7,7 @@
 import numpy as np
 import prometheus_client
 
-from vllm.config import VllmConfig
+from vllm.config import SupportsMetricsInfo, VllmConfig
 from vllm.logger import init_logger
 from vllm.v1.core.kv_cache_utils import PrefixCachingMetrics
 from vllm.v1.engine import FinishReason
@@ -228,6 +228,26 @@ def __init__(self, vllm_config: VllmConfig):
                 buckets=request_latency_buckets,
                 labelnames=labelnames).labels(*labelvalues)
 
+        self.log_metrics_info("cache_config", vllm_config.cache_config)
+
+    def log_metrics_info(self, type: str, config_obj: SupportsMetricsInfo):
+        metrics_info = config_obj.metrics_info()
+
+        name, documentation = None, None
+        if type == "cache_config":
+            name = "vllm:cache_config_info"
+            documentation = "Information of the LLMEngine CacheConfig"
+        assert name is not None, f"Unknown metrics info type {type}"
+
+        # Info type metrics are syntactic sugar for a gauge permanently set to 1
+        # Since prometheus multiprocessing mode does not support Info, emulate
+        # info here with a gauge.
+        info_gauge = prometheus_client.Gauge(
+            name=name,
+            documentation=documentation,
+            labelnames=metrics_info.keys()).labels(**metrics_info)
+        info_gauge.set(1)
+
     def log(self, scheduler_stats: SchedulerStats,
             iteration_stats: IterationStats):
         """Log to prometheus."""