Multiproc doesn't capture all metrics

Hey folks,

I have a similar setup to what is described here. The only difference is that I have nginx in front talking to gunicorn over sockets, but I don't think that matters as far as I can tell.

I wasn't able to use this code:

# Using multiprocess collector for registry
def make_metrics_app():
    registry = CollectorRegistry()
    multiprocess.MultiProcessCollector(registry)
    return make_asgi_app(registry=registry)


metrics_app = make_metrics_app()
app.mount("/metrics", metrics_app)

It results in a page that gets a 307, so I implemented this:

@router.get("/metrics", response_class=PlainTextResponse, include_in_schema=False)
async def get_metrics() -> str:
    """
    Get prometheus metrics
    """
    registry = CollectorRegistry()
    multiprocess.MultiProcessCollector(registry)
    data = generate_latest(registry)
    res = Response(content=data)
    res.headers["Content-Type"] = CONTENT_TYPE_LATEST
    res.headers["Content-Length"] = str(len(data))
    return res

Apart from that my setup is pretty typical, I have some counters and a histogram:

AUTOMATIONS_TOTAL: Final = Gauge(
    name=prefix_("automations_total"),
    documentation="Number of automations run",
    labelnames=["platform", "action", "kind", "status"],
    multiprocess_mode="sum",
)

REQUEST_TIME_SECONDS: Final = Histogram(
    name=prefix_("request_time_seconds"),
    documentation="Time spent processing request",
    labelnames=["method", "url_rule", "status_code"],
)

REQUESTS_IN_PROGRESS_TOTAL: Final = Gauge(
    name=prefix_("requests_in_progress_total"),
    documentation="Number of concurrent requests",
    # See Metrics Tuning (Gauge)
    # https://github.com/prometheus/client_python#multiprocess-mode-gunicorn
    multiprocess_mode="sum",
)

My in progress counter always remains at zero, but I'm hoping that's because requests finish faster than I can see them. The histogram, however, never shows up - I see no record of it in my PROMETHEUS_MULTIPROC_DIR and it's never rendered in the output. Any ideas for what I could troubleshoot?

Start by reproducing the FastAPI multiprocess collector path and the custom /metrics handler shown in the issue. Inspect CollectorRegistry, MultiProcessCollector, and the PROMETHEUS_MULTIPROC_DIR output while comparing the counter, gauge, and histogram behavior. Done means identifying why the histogram is absent and confirming the expected metrics are rendered.