diff --git a/synapse/util/metrics.py b/synapse/util/metrics.py
index bc3b4938ea..165480bdbe 100644
--- a/synapse/util/metrics.py
+++ b/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, Dict, Generator, 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: Dict[str, 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.values():
+ pre_update_hook()
+
+ yield from super().collect()
+
+ def register_hook(self, metric_name: str, hook: Callable[[], None]) -> None:
+ """
+ Registers a hook that is called before metric collection.
+ """
+
+ self._pre_update_hooks[metric_name] = hook
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