[V1][Metrics] Add initial Prometheus logger (#12416)

Signed-off-by: Mark McLoughlin <markmc@redhat.com>

vllm/v1/engine/async_llm.py

@@ -24,7 +24,8 @@ from vllm.v1.engine.core_client import EngineCoreClient
 from vllm.v1.engine.output_processor import OutputProcessor
 from vllm.v1.engine.processor import Processor
 from vllm.v1.executor.abstract import Executor
-from vllm.v1.metrics.loggers import LoggingStatLogger, StatLoggerBase
+from vllm.v1.metrics.loggers import (LoggingStatLogger, PrometheusStatLogger,
+                                     StatLoggerBase)
 from vllm.v1.metrics.stats import IterationStats, SchedulerStats
 
 logger = init_logger(__name__)
@@ -46,13 +47,15 @@ class AsyncLLM(EngineClient):
 
         assert start_engine_loop
 
+        self.model_config = vllm_config.model_config
+
         self.log_requests = log_requests
         self.log_stats = log_stats
         self.stat_loggers: List[StatLoggerBase] = [
             LoggingStatLogger(),
-            # TODO(rob): PrometheusStatLogger(),
+            PrometheusStatLogger(labels=dict(
+                model_name=self.model_config.served_model_name)),
         ]
-        self.model_config = vllm_config.model_config
 
         # Tokenizer (+ ensure liveness if running in another process).
         self.tokenizer = init_tokenizer_from_configs(
@@ -272,7 +275,7 @@ class AsyncLLM(EngineClient):
 
                 # 4) Logging.
                 # TODO(rob): make into a coroutine and launch it in
-                # background thread once we add Prometheus.
+                # background thread once Prometheus overhead is non-trivial.
                 assert iteration_stats is not None
                 self._log_stats(
                     scheduler_stats=outputs.scheduler_stats,