Re-introduce: Fix LaterGauge metrics to collect from all servers (#18791)

Re-introduce: https://github.com/element-hq/synapse/pull/18751 that was
reverted in https://github.com/element-hq/synapse/pull/18789 (explains
why the PR was reverted in the first place).

- Adds a `cleanup` pattern that cleans up metrics from each homeserver
in the tests. Previously, the list of hooks built up until our CI
machines couldn't operate properly, see
https://github.com/element-hq/synapse/pull/18789
- Fix long-standing issue with `synapse_background_update_status`
metrics only tracking the last database listed in the config (see
https://github.com/element-hq/synapse/pull/18791#discussion_r2261706749)

synapse/_scripts/generate_workers_map.py

@@ -153,9 +153,13 @@ def get_registered_paths_for_default(
     """
 
     hs = MockHomeserver(base_config, worker_app)
+
     # TODO We only do this to avoid an error, but don't need the database etc
     hs.setup()
-    return get_registered_paths_for_hs(hs)
+    registered_paths = get_registered_paths_for_hs(hs)
+    hs.cleanup()
+
+    return registered_paths
 
 
 def elide_http_methods_if_unconflicting(

synapse/_scripts/synapse_port_db.py

@@ -99,6 +99,7 @@ from synapse.storage.engines import create_engine
 from synapse.storage.prepare_database import prepare_database
 from synapse.types import ISynapseReactor
 from synapse.util import SYNAPSE_VERSION, Clock
+from synapse.util.stringutils import random_string
 
 # Cast safety: Twisted does some naughty magic which replaces the
 # twisted.internet.reactor module with a Reactor instance at runtime.
@@ -323,6 +324,7 @@ class MockHomeserver:
         self.config = config
         self.hostname = config.server.server_name
         self.version_string = SYNAPSE_VERSION
+        self.instance_id = random_string(5)
 
     def get_clock(self) -> Clock:
         return self.clock
@@ -330,6 +332,9 @@ class MockHomeserver:
     def get_reactor(self) -> ISynapseReactor:
         return reactor
 
+    def get_instance_id(self) -> str:
+        return self.instance_id
+
     def get_instance_name(self) -> str:
         return "master"
 
@@ -685,7 +690,15 @@ class Porter:
             )
             prepare_database(db_conn, engine, config=self.hs_config)
             # Type safety: ignore that we're using Mock homeservers here.
-            store = Store(DatabasePool(hs, db_config, engine), db_conn, hs)  # type: ignore[arg-type]
+            store = Store(
+                DatabasePool(
+                    hs,  # type: ignore[arg-type]
+                    db_config,
+                    engine,
+                ),
+                db_conn,
+                hs,  # type: ignore[arg-type]
+            )
             db_conn.commit()
 
         return store