fix: mount FastStream metrics for an injected collector registry

docs/introduction/configuration.md

@@ -73,9 +73,18 @@ Additional parameters for Litestar integration:
 
 Prometheus's integration for FastStream requires `prometheus_client` package.
 
-To bootstrap Prometheus for FastStream, you must provide additionally:
-
-- `prometheus_middleware_cls`.
+To bootstrap Prometheus for FastStream, you must provide at least one of:
+
+- `prometheus_middleware_cls` - the broker metrics middleware, e.g.
+  `faststream.redis.prometheus.RedisPrometheusMiddleware`. It is constructed with the instrument's
+  registry and added to the broker, when the application has one.
+- `prometheus_collector_registry` - a `prometheus_client.CollectorRegistry` of your own, used in
+  place of the fresh one the instrument would otherwise build.
+
+With neither, nothing would populate the registry, so the metrics endpoint is not mounted and the
+instrument is skipped. Unlike the other frameworks, FastStream serves a private registry rather than
+`prometheus_client.REGISTRY`, so an endpoint with no middleware and no injected registry would have
+nothing to report.
 
 ### Prometheus FastAPI
 

lite_bootstrap/bootstrappers/faststream_bootstrapper.py

@@ -161,7 +161,10 @@ def _make_collector_registry() -> "prometheus_client.CollectorRegistry":
 class FastStreamPrometheusInstrument(PrometheusInstrument):
     bootstrap_config: FastStreamConfig
     collector_registry: "prometheus_client.CollectorRegistry" = dataclasses.field(init=False)
-    not_configured_reason = PrometheusInstrument.not_configured_reason + " or prometheus_middleware_cls is missing"
+    not_configured_reason = (
+        PrometheusInstrument.not_configured_reason
+        + ", or neither prometheus_middleware_cls nor prometheus_collector_registry is set"
+    )
     missing_dependency_message = "prometheus_client is not installed"
 
     def __post_init__(self) -> None:
@@ -170,7 +173,10 @@ def __post_init__(self) -> None:
 
     @classmethod
     def is_configured(cls, bootstrap_config: "FastStreamConfig") -> bool:  # ty: ignore[invalid-method-override]
-        return super().is_configured(bootstrap_config) and bool(bootstrap_config.prometheus_middleware_cls)
+        return super().is_configured(bootstrap_config) and (
+            bootstrap_config.prometheus_middleware_cls is not None
+            or bootstrap_config.prometheus_collector_registry is not None
+        )
 
     @staticmethod
     def dependencies_installed() -> bool:

tests/test_faststream_bootstrap.py

@@ -23,6 +23,7 @@
 from lite_bootstrap.bootstrappers.faststream_bootstrapper import (
     FastStreamLoggingInstrument,
     FastStreamOpenTelemetryInstrument,
+    FastStreamPrometheusInstrument,
 )
 from lite_bootstrap.exceptions import ConfigurationError
 from tests.conftest import (
@@ -254,14 +255,18 @@ def test_faststream_build_excluded_urls_covers_prometheus_and_health_paths(broke
     assert config_with_health_spans.health_checks_path not in excluded_with  # kept when health spans are on
 
 
-async def test_faststream_prometheus_uses_injected_registry(broker: RedisBroker) -> None:
+@pytest.mark.parametrize(
+    "middleware_cls", [RedisPrometheusMiddleware, None], ids=["with_middleware", "without_middleware"]
+)
+async def test_faststream_prometheus_uses_injected_registry(broker: RedisBroker, middleware_cls: type | None) -> None:
     custom_registry = prometheus_client.CollectorRegistry()
     counter_name = f"injected_counter_{uuid.uuid4().hex}_total"
     counter = prometheus_client.Counter(counter_name, "Injected registry counter", registry=custom_registry)
     counter.inc()
 
     bootstrap_config = dataclasses.replace(
         build_faststream_config(broker=broker),
+        prometheus_middleware_cls=middleware_cls,
         prometheus_collector_registry=custom_registry,
     )
     bootstrapper = FastStreamBootstrapper(bootstrap_config=bootstrap_config)
@@ -276,6 +281,27 @@ async def test_faststream_prometheus_uses_injected_registry(broker: RedisBroker)
         bootstrapper.teardown()
 
 
+async def test_faststream_prometheus_is_skipped_without_middleware_or_registry(broker: RedisBroker) -> None:
+    bootstrap_config = dataclasses.replace(
+        build_faststream_config(broker=broker),
+        prometheus_middleware_cls=None,
+        prometheus_collector_registry=None,
+    )
+    bootstrapper = FastStreamBootstrapper(bootstrap_config=bootstrap_config)
+    skipped = dict(bootstrapper.skipped_instruments)
+    assert FastStreamPrometheusInstrument in skipped
+    assert "prometheus_collector_registry" in skipped[FastStreamPrometheusInstrument]
+
+    application = bootstrapper.bootstrap()
+    try:
+        with TestClient(app=application) as test_client:
+            async with TestRedisBroker(broker):
+                response = test_client.get(bootstrap_config.prometheus_metrics_path)
+                assert response.status_code == status.HTTP_404_NOT_FOUND
+    finally:
+        bootstrapper.teardown()
+
+
 def test_faststream_logging_teardown_runs_super_when_broker_write_raises(broker: RedisBroker) -> None:
     bootstrap_config = build_faststream_config(broker=broker)
     instrument = FastStreamLoggingInstrument(bootstrap_config=bootstrap_config)