Don't warn about a missing Prometheus extra while the metrics path is still the default #398

PrometheusConfig.prometheus_metrics_path defaults to /metrics (lite_bootstrap/instruments/prometheus_instrument.py:12), so the Prometheus instrument counts as configured on FastAPI, Litestar and FastMCP even when the user never mentions Prometheus. A service installed without the -metrics extra then gets an InstrumentDependencyMissingWarning and a logged warning every time a bootstrapper is constructed (bootstrappers/base.py:66-77). configuration.md says Prometheus "is on by default when its extra is installed", so a missing extra should be a quiet opt-out.

Found in the 2.0 audit.

Decision: the defaults stay

The audit asked whether every framework should share one default. They keep the ones they have:

• FastAPI, Litestar and FastMCP keep /metrics. Making it opt-in would break every service whose scraper expects /metrics without config, and nothing would fail at startup.
• FastStream stays opt-in. It already was in 1.16.0, where the instrument needed prometheus_middleware_cls or prometheus_collector_registry (faststream_bootstrapper.py:217-221 at the tag); FastStream Prometheus: use the default registry and make the metrics path opt-in #375 moved the switch to the path. Broker metrics need a middleware class specific to the broker, which lite-bootstrap can't pick, so a default /metrics would serve only process metrics.

Work

• When prometheus_metrics_path still has its default value and prometheus_client is missing, skip the instrument silently, as a not-configured skip. Keep the warning when the user set the path to anything else.
• A user who sets "/metrics" explicitly can't be told apart from the default, so they get no warning. Accept that gap rather than adding a sentinel default.
• Say in the Prometheus section of configuration.md why FastStream is opt-in and the other frameworks are not.
• Test first: on FastAPI with prometheus_client hidden, constructing the bootstrapper with default config emits no warning, and with prometheus_metrics_path="/custom" it still warns.

Breaking changes

None. A warning goes away, and no default changes.