fix(prometheus): answer /metrics without a trailing-slash redirect

app.mount("/metrics", ...) makes starlette answer the bare path with
307 Location: /metrics/. Prometheus and grafana alloy do not follow redirects,
so the scrape reads an empty body while up stays 1 - metric loss with nothing
logged.

Register both spellings as routes. ASGIRoute hands the raw ASGI triple to the
metrics app so its streaming, gzip and render-coalescing survive, instead of
buffering the whole scrape to build one Response.

Signed-off-by: hclsys <chenglunhu@gmail.com>

litellm/integrations/prometheus.py

@@ -4553,7 +4553,10 @@ class PrometheusLogger(CustomLogger):
         from prometheus_client import REGISTRY
 
         from litellm._logging import verbose_proxy_logger
-        from litellm.integrations.prometheus_metrics_endpoint import make_metrics_asgi_app
+        from litellm.integrations.prometheus_metrics_endpoint import (
+            ASGIRoute,
+            make_metrics_asgi_app,
+        )
         from litellm.proxy.proxy_server import app
 
         # Create metrics ASGI app
@@ -4566,8 +4569,20 @@ class PrometheusLogger(CustomLogger):
         else:
             metrics_app = make_metrics_asgi_app(REGISTRY)
 
-        # Mount the metrics app to the app
-        app.mount("/metrics", metrics_app)
+        # `app.mount("/metrics", ...)` makes starlette answer the bare `/metrics`
+        # with `307 Location: /metrics/`. Prometheus and grafana alloy do not
+        # follow redirects, so the scrape reads an empty body while `up` stays 1
+        # -- metric loss with no error anywhere (#30079).
+        #
+        # Register both spellings as routes instead. `ASGIRoute` is what keeps
+        # `metrics_app`'s streaming, gzip and render-coalescing intact -- a plain
+        # function endpoint would have to buffer the whole scrape to hand back a
+        # single Response.
+        # `methods` is left at its default: starlette's `Route` resolves None to
+        # exactly `["GET"]`, so naming it here would only add a mutable literal.
+        metrics_route: Final = ASGIRoute(metrics_app)
+        app.add_route("/metrics", metrics_route)
+        app.add_route("/metrics/", metrics_route)
         verbose_proxy_logger.debug("Starting Prometheus Metrics on /metrics (no authentication)")
 
 

litellm/integrations/prometheus_metrics_endpoint.py

@@ -79,6 +79,22 @@ def _chunks(body: bytes) -> Iterator[bytes]:
     return (body[start : start + RESPONSE_CHUNK_SIZE_BYTES] for start in range(0, len(body), RESPONSE_CHUNK_SIZE_BYTES))
 
 
+class ASGIRoute:
+    """Adapt an ASGI app so it can be registered with ``Starlette.add_route``.
+
+    Starlette calls a plain function endpoint as ``f(request)`` and expects a
+    ``Response`` back. A callable object is handed the raw ASGI triple instead,
+    which lets the mounted app write its own streaming response rather than
+    having the whole scrape buffered to build one.
+    """
+
+    def __init__(self, app: ASGIApp) -> None:
+        self._app = app
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        await self._app(scope, receive, send)
+
+
 def make_metrics_asgi_app(registry: CollectorRegistry) -> ASGIApp:
     renderer: Final = CoalescedScrapeRenderer(registry)
 

tests/test_litellm/integrations/test_prometheus_metrics_endpoint.py

@@ -274,3 +274,49 @@ async def test_a_finishing_render_does_not_evict_another_that_is_still_in_flight
     assert collector.collect_calls == 1, "an unrelated render finishing evicted the render still in flight"
     for response in responses:
         assert b"gated_metric" in response.content
+
+
+def test_both_metrics_paths_answer_without_a_redirect(monkeypatch: pytest.MonkeyPatch):
+    """`app.mount("/metrics", ...)` answers the bare path with 307 Location:
+    /metrics/. Prometheus and grafana alloy do not follow redirects, so the
+    scrape reads an empty body while `up` stays 1 (#30079). Both spellings must
+    return the payload directly.
+
+    Drives the real `_mount_metrics_endpoint` against a stand-in app so the
+    wiring itself is covered, not just `ASGIRoute` in isolation.
+    """
+    import sys
+    import types
+
+    from fastapi import FastAPI
+    from fastapi.testclient import TestClient
+
+    # Import before the stub goes in. `_mount_metrics_endpoint` resolves
+    # `litellm.proxy.proxy_server` lazily, and standing a fake module in that
+    # slot while this module graph is still being built changes what the real
+    # imports underneath it resolve to.
+    from litellm.integrations.prometheus import PrometheusLogger
+
+    app: Final = FastAPI()
+    proxy_server: Final = types.ModuleType("litellm.proxy.proxy_server")
+    proxy_server.app = app  # type: ignore[attr-defined]
+    monkeypatch.setitem(sys.modules, "litellm.proxy.proxy_server", proxy_server)
+
+    PrometheusLogger._mount_metrics_endpoint()
+
+    assert {route.path for route in app.router.routes if "metrics" in getattr(route, "path", "")} == {
+        "/metrics",
+        "/metrics/",
+    }
+
+    client: Final = TestClient(app, follow_redirects=False)
+    for path in ("/metrics", "/metrics/"):
+        response = client.get(path)
+        # 200 rather than the 307 the mount used to emit. What the registry
+        # holds is deliberately not asserted -- this shares the process-wide
+        # default REGISTRY, so another test may legitimately have drained it.
+        assert response.status_code == 200, path
+        assert response.headers["content-type"].startswith("text/plain"), path
+
+    # The streaming app still owns the response, so its behaviour survives.
+    assert client.get("/metrics", headers={"accept-encoding": "gzip"}).headers["content-encoding"] == "gzip"