fix(proxy): point /metrics 401 at the opt-out flag

Operators upgrading past 35bbca60b0 (which made /metrics auth
default-on) see "Malformed API Key passed in. Ensure Key has 'Bearer '
prefix." with no hint that
litellm_settings.require_auth_for_metrics_endpoint: false restores the
previous unauthenticated behavior. Append that discovery hint to the
existing 401 body so a Prometheus scraper that breaks after upgrade
has a clear migration path. No behavior change.

litellm/proxy/middleware/prometheus_auth_middleware.py

@@ -79,7 +79,10 @@ class PrometheusAuthMiddleware:
                 # Send 401 response directly via ASGI protocol
                 error_message = getattr(e, "message", str(e))
                 body = json.dumps(
-                    f"Unauthorized access to metrics endpoint: {error_message}"
+                    f"Unauthorized access to metrics endpoint: {error_message} "
+                    f"To allow unauthenticated access, set "
+                    f"`litellm_settings.require_auth_for_metrics_endpoint: false` "
+                    f"in your proxy_config.yaml."
                 ).encode("utf-8")
                 await send(
                     {

tests/test_litellm/proxy/middleware/test_prometheus_auth_middleware.py

@@ -121,6 +121,26 @@ def test_invalid_auth_metrics(app_with_middleware, monkeypatch):
     assert "Unauthorized access to metrics endpoint" in response.text
 
 
+def test_invalid_auth_metrics_includes_optout_hint(app_with_middleware, monkeypatch):
+    """
+    The 401 body must tell operators how to restore the previous unauthenticated
+    behavior, otherwise a Prometheus scraper that worked pre-upgrade just sees
+    "Malformed API Key" with no actionable migration path.
+    """
+    monkeypatch.setattr(litellm, "require_auth_for_metrics_endpoint", True)
+    monkeypatch.setattr(
+        "litellm.proxy.middleware.prometheus_auth_middleware.user_api_key_auth",
+        fake_invalid_auth,
+    )
+
+    client = TestClient(app_with_middleware)
+    response = client.get("/metrics")
+
+    assert response.status_code == 401, response.text
+    assert "require_auth_for_metrics_endpoint" in response.text
+    assert "false" in response.text
+
+
 def test_metrics_auth_uses_real_auth_when_route_is_public(
     app_with_middleware, monkeypatch
 ):