Don't enforce the test for the hack on old prometheus-client versions…

… before it was possible

tests/test_metrics.py

@@ -14,6 +14,9 @@
 # limitations under the License.
 from unittest.mock import Mock, patch
 
+import importlib_metadata
+from pkg_resources import parse_version
+
 from synapse.app._base import _set_prometheus_client_use_created_metrics
 from synapse.metrics import REGISTRY, InFlightGauge, generate_latest
 from synapse.util.caches.deferred_cache import DeferredCache
@@ -167,6 +170,11 @@ def test_cache_metric(self):
 
 
 class PrometheusMetricsHackTestCase(unittest.HomeserverTestCase):
+    if parse_version(importlib_metadata.version("prometheus_client")) < parse_version(
+        "0.14.0"
+    ):
+        skip = "prometheus-client too old"
+
     def test_created_metrics_disabled(self) -> None:
         """
         Tests that a brittle hack, to disable `_created` metrics, works.