FastStream mounts no /metrics endpoint when prometheus_middleware_cls is unset, even with an injected registry #229

Noticed while reviewing #226. Same shape, but not the same fix: deleting the override here would be wrong.

The gate

@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)

while bootstrap() mounts the endpoint unconditionally and only the middleware is guarded:

def bootstrap(self) -> None:
    config = self.bootstrap_config
    config.application.mount(
        config.prometheus_metrics_path, prometheus_client.make_asgi_app(self.collector_registry)
    )
    if config.prometheus_middleware_cls and config.application.broker:
        config.application.broker.add_middleware(config.prometheus_middleware_cls(registry=self.collector_registry))

So without prometheus_middleware_cls the instrument is never instantiated and the /metrics mount
never happens, silently (_select_instruments records config-level skips without warning, by design).

The case that is actually lost

FastStreamConfig accepts prometheus_collector_registry, and __post_init__ uses an injected
registry in preference to a fresh one. A service that injects a registry holding its own collectors
and does not use the broker middleware gets no endpoint to scrape them from. The registry is accepted
and does nothing, which is the #226 complaint in a different instrument.

Why the #226 fix does not transfer

PrometheusConfig.prometheus_metrics_path defaults to "/metrics", and the base gate is
bool(path) and is_valid_path(path). That is true by default. The prometheus_middleware_cls
override is therefore the only thing stopping every FastStream application from mounting a
/metrics endpoint. Deleting it the way #226 deleted the OpenTelemetry override would change the
HTTP surface of every FastStream service that never asked for Prometheus.

The OpenTelemetry case had no such problem: its base gate is
bool(opentelemetry_endpoint or opentelemetry_log_traces), both defaulting to off, so the user had
to opt in before anything happened.

Options, none obviously right

1. Gate on the registry instead: configured when prometheus_middleware_cls or
   prometheus_collector_registry is set. Narrow, fixes exactly the lost case, leaves default apps
   untouched. Slightly odd as a rule to explain.
2. Make the path opt-in on FastStream by defaulting prometheus_metrics_path to "" there, then
   delete the override for real symmetry with FastStream skips the whole OpenTelemetry instrument when opentelemetry_middleware_cls is unset #226. Cleanest rule, but a breaking change for anyone
   relying on the default path today.
3. Leave it and document it, on the grounds that FastStream Prometheus support means the broker
   middleware and nothing else. Then prometheus_collector_registry should say so.

Worth deciding rather than defaulting, which is why this is filed rather than fixed.