Fix cache metrics not being updated when not using the legacy exposition module. (#13717)

synapse/util/metrics.py

@@ -15,9 +15,9 @@
 import logging
 from functools import wraps
 from types import TracebackType
-from typing import Awaitable, Callable, Optional, Type, TypeVar
+from typing import Awaitable, Callable, Generator, List, Optional, Type, TypeVar
 
-from prometheus_client import Counter
+from prometheus_client import CollectorRegistry, Counter, Metric
 from typing_extensions import Concatenate, ParamSpec, Protocol
 
 from synapse.logging.context import (
@@ -208,3 +208,33 @@ class Measure:
         metrics.real_time_sum += duration
 
         # TODO: Add other in flight metrics.
+
+
+class DynamicCollectorRegistry(CollectorRegistry):
+    """
+    Custom Prometheus Collector registry that calls a hook first, allowing you
+    to update metrics on-demand.
+
+    Don't forget to register this registry with the main registry!
+    """
+
+    def __init__(self) -> None:
+        super().__init__()
+        self._pre_update_hooks: List[Callable[[], None]] = []
+
+    def collect(self) -> Generator[Metric, None, None]:
+        """
+        Collects metrics, calling pre-update hooks first.
+        """
+
+        for pre_update_hook in self._pre_update_hooks:
+            pre_update_hook()
+
+        yield from super().collect()
+
+    def register_hook(self, hook: Callable[[], None]) -> None:
+        """
+        Registers a hook that is called before metric collection.
+        """
+
+        self._pre_update_hooks.append(hook)