Restore _set_prometheus_client_use_created_metrics

See https://github.com/element-hq/synapse/pull/18584#discussion_r2159670686

synapse/metrics/_twisted_exposition.py

@@ -20,6 +20,7 @@
 #
 #
 
+import logging
 from prometheus_client import (
     REGISTRY,
     CollectorRegistry,
@@ -30,14 +31,34 @@ from prometheus_client import (
 from twisted.web.resource import Resource
 from twisted.web.server import Request
 
+logger = logging.getLogger(__name__)
+
 CONTENT_TYPE_LATEST = "text/plain; version=0.0.4; charset=utf-8"
 
-# Sets whether `prometheus_client` should expose `_created`-suffixed metrics for all
-# gauges, histograms and summaries.
-#
-# The motivation for disabling these `_created` metrics is that they're a waste of space
-# as they're not useful but they take up space in Prometheus.
-disable_created_metrics()
+
+def _set_prometheus_client_use_created_metrics(new_value: bool) -> None:
+    """
+    Sets whether prometheus_client should expose `_created`-suffixed metrics for
+    all gauges, histograms and summaries.
+
+    There is no programmatic way to disable this without poking at internals;
+    the proper way is to use an environment variable which prometheus_client
+    loads at import time.
+
+    The motivation for disabling these `_created` metrics is that they're
+    a waste of space as they're not useful but they take up space in Prometheus.
+    """
+    import prometheus_client.metrics
+
+    if hasattr(prometheus_client.metrics, "_use_created"):
+        prometheus_client.metrics._use_created = new_value
+    else:
+        logger.error(
+            "Can't disable `_created` metrics in prometheus_client (brittle hack broken?)"
+        )
+
+
+_set_prometheus_client_use_created_metrics(False)
 
 
 class MetricsResource(Resource):