Register the media threadpool with our metrics

Otherwise it won't be visible.

synapse/server.py

@@ -124,6 +124,7 @@
 )
 from synapse.http.matrixfederationclient import MatrixFederationHttpClient
 from synapse.media.media_repository import MediaRepository
+from synapse.metrics import register_threadpool
 from synapse.metrics.common_usage_metrics import CommonUsageMetricsManager
 from synapse.module_api import ModuleApi
 from synapse.module_api.callbacks import ModuleApiCallbacks
@@ -959,4 +960,7 @@ def get_media_sender_thread_pool(self) -> ThreadPool:
             "during", "shutdown", media_threadpool.stop
         )
 
+        # Register the threadpool with our metrics.
+        register_threadpool("media", media_threadpool)
+
         return media_threadpool