fix: quietly skip default Prometheus without client

CONTEXT.md

@@ -29,7 +29,8 @@ per-framework guides, and Sentry's word for its own `Integration` objects.
 **Configured**:
 An instrument is configured when the user's config asks for it: `is_configured(config)`, a
 classmethod evaluated before anything is instantiated. Unconfigured is the normal case, not a fault:
-an empty `sentry_dsn` means the user did not want Sentry.
+an empty `sentry_dsn` means the user did not want Sentry. Prometheus at its default metrics path
+is also unconfigured when `prometheus_client` is missing, treating an absent extra as an opt-out.
 _Avoid_: enabled. `logging_enabled` and `health_checks_enabled` are two config fields among the
 several inputs `is_configured` reads; most instruments have no enabled flag at all.
 
@@ -45,7 +46,8 @@ An instrument the bootstrapper decided not to instantiate. Two paths, deliberate
 signals: not configured is silent and lands in `skipped_instruments`, so it shows in
 `build_summary()`; configured but with its optional package missing is a deployment surprise, so it
 raises `InstrumentDependencyMissingWarning` plus a `logger.warning` and lands in neither list. The
-quiet skip is the one you have to go looking for; the loud one finds you.
+quiet skip includes Prometheus at its default path without `prometheus_client`; a custom path
+with the same missing dependency takes the warning path.
 
 **Health probe**:
 One check of something the service depends on, run each time the health check route is requested. A

docs/introduction/configuration.md

@@ -119,9 +119,15 @@ the option [here](https://docs.sentry.io/platforms/python/data-management/data-c
 
 Prometheus is the cheapest of the three non-logging instruments; see [Performance](performance.md).
 
-Prometheus is on by default when its extra is installed, serving metrics at `prometheus_metrics_path`
-(default: `/metrics`). Set `prometheus_metrics_path` to an empty string to disable it. FastStream is
-the exception: its default is empty; see [Prometheus FastStream](#prometheus-faststream).
+On FastAPI, Litestar and FastMCP, Prometheus is on by default when its extra is installed,
+serving metrics at `prometheus_metrics_path` (default: `/metrics`). Set the path to an empty
+string to disable it. Without `prometheus_client`, the default path is a silent opt-out;
+a custom path still emits a missing-dependency warning. Explicitly setting `/metrics`
+is indistinguishable from leaving the default and is also silent.
+
+FastStream is opt-in: its default path is empty because broker metrics require a
+broker-specific middleware class that lite-bootstrap cannot choose automatically.
+See [Prometheus FastStream](#prometheus-faststream).
 
 Additional parameters:
 

lite_bootstrap/bootstrappers/fastapi_bootstrapper.py

@@ -245,7 +245,10 @@ class FastAPIPrometheusInstrument(PrometheusInstrument[FastAPIConfig]):
 
     @staticmethod
     def dependencies_installed() -> bool:
-        return import_checker.is_prometheus_fastapi_instrumentator_installed
+        return (
+            import_checker.is_prometheus_client_installed
+            and import_checker.is_prometheus_fastapi_instrumentator_installed
+        )
 
     def bootstrap(self) -> None:
         config = self.bootstrap_config

lite_bootstrap/instruments/prometheus_instrument.py

@@ -4,6 +4,7 @@
 import re
 import typing
 
+from lite_bootstrap import import_checker
 from lite_bootstrap.exceptions import ConfigurationError
 from lite_bootstrap.helpers.path import is_valid_path
 from lite_bootstrap.instruments.base import BaseConfig, BaseInstrument
@@ -38,11 +39,15 @@ def is_prometheus_http_metrics_on(self, *, opentelemetry_records_http_metrics: b
 
 @dataclasses.dataclass(kw_only=True, slots=True)
 class PrometheusInstrument(BaseInstrument[PrometheusConfigT]):
-    not_configured_reason = "prometheus_metrics_path is empty"
+    not_configured_reason = "prometheus_metrics_path is empty or uses its default without prometheus_client"
 
     @classmethod
     def is_configured(cls, bootstrap_config: "PrometheusConfig") -> bool:
-        return bool(bootstrap_config.prometheus_metrics_path)
+        path = bootstrap_config.prometheus_metrics_path
+        default_path = next(
+            field.default for field in dataclasses.fields(bootstrap_config) if field.name == "prometheus_metrics_path"
+        )
+        return bool(path) and (path != default_path or import_checker.is_prometheus_client_installed)
 
     def infrastructure_excluded_patterns(self) -> list[str]:
         config = self.bootstrap_config

tests/instruments/test_prometheus_instrument.py

@@ -9,7 +9,10 @@ def test_prometheus_instrument_configured_with_default_path() -> None:
 def test_prometheus_instrument_not_configured_with_empty_path() -> None:
     config = PrometheusConfig(prometheus_metrics_path="")
     assert not PrometheusInstrument.is_configured(config)
-    assert PrometheusInstrument.not_configured_reason == "prometheus_metrics_path is empty"
+    assert (
+        PrometheusInstrument.not_configured_reason
+        == "prometheus_metrics_path is empty or uses its default without prometheus_client"
+    )
 
 
 def test_prometheus_instrument_configured_with_custom_valid_path() -> None:

tests/test_fastapi_bootstrap.py

@@ -253,6 +253,34 @@ def test_fastapi_bootstrapper_with_missing_instrument_dependency(
         FastAPIBootstrapper(bootstrap_config=fastapi_config)
 
 
+@pytest.mark.parametrize("metrics_path", [None, "/custom"])
+def test_fastapi_missing_prometheus_is_quiet_only_for_the_default_path(
+    metrics_path: str | None, caplog: pytest.LogCaptureFixture
+) -> None:
+    config = FastAPIConfig() if metrics_path is None else FastAPIConfig(prometheus_metrics_path=metrics_path)
+    with emulate_package_missing("prometheus_client"), warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        bootstrapper = FastAPIBootstrapper(bootstrap_config=config)
+    try:
+        assert not any(
+            isinstance(instrument, fastapi_bootstrapper.FastAPIPrometheusInstrument)
+            for instrument in bootstrapper.instruments
+        )
+        if metrics_path is None:
+            assert not caught
+            assert not caplog.records
+            assert any(
+                instrument is fastapi_bootstrapper.FastAPIPrometheusInstrument
+                for instrument, _reason in bootstrapper.skipped_instruments
+            )
+        else:
+            assert [warning.category for warning in caught] == [InstrumentDependencyMissingWarning]
+            assert "prometheus" in str(caught[0].message)
+            assert "prometheus" in caplog.text
+    finally:
+        bootstrapper.teardown()
+
+
 def test_fastapi_bootstrapper_runs_without_sentry_sdk(fastapi_config: FastAPIConfig) -> None:
     with emulate_package_missing_with_module_reload(
         "sentry_sdk",

tests/test_fastmcp_bootstrap.py

@@ -341,7 +341,7 @@ def test_fastmcp_logging_config_warns_nothing(overrides: dict[str, bool]) -> Non
     [
         ("sentry_sdk", {"sentry_dsn": "https://testdsn@localhost/1"}),
         ("structlog", {}),
-        ("prometheus_client", {}),
+        ("prometheus_client", {"prometheus_metrics_path": "/custom"}),
     ],
 )
 def test_fastmcp_bootstrapper_with_missing_instrument_dependency(
@@ -372,8 +372,10 @@ def test_fastmcp_bootstrap_without_prometheus_client() -> None:
         "prometheus_client",
         ["lite_bootstrap.bootstrappers.fastmcp_bootstrapper"],
     ):
-        with pytest.warns(UserWarning, match="prometheus_client"):
+        with warnings.catch_warnings(record=True) as caught:
+            warnings.simplefilter("always")
             bootstrapper = FastMcpBootstrapper(bootstrap_config=FastMcpConfig())
+        assert not caught
         bootstrapper.bootstrap()
         bootstrapper.teardown()
 

tests/test_faststream_bootstrap.py

@@ -512,7 +512,7 @@ def test_faststream_prometheus_is_skipped_without_metrics_path(broker: RedisBrok
     )
     bootstrapper = FastStreamBootstrapper(bootstrap_config=bootstrap_config)
     skipped = dict(bootstrapper.skipped_instruments)
-    assert skipped[FastStreamPrometheusInstrument] == "prometheus_metrics_path is empty"
+    assert skipped[FastStreamPrometheusInstrument] == FastStreamPrometheusInstrument.not_configured_reason
 
 
 async def test_faststream_prometheus_serves_the_default_registry(broker: RedisBroker) -> None: