matrix: report homeserver up, latency and database size through existing collectors

tuwunel 1.9.4 exports no metrics. The host collector gets an httpcheck
receiver on the gateway's /_matrix/client/versions (new
services.hyperhive.otel.httpChecks, declared by hive-matrix), and a timer in
the hive-matrix container pushes the database directory's size through the
forwarder's OTLP port, as the attic cache-size unit does. The size step
dereferences the state directory's symlink and pushes nothing for a result
under 4096 bytes.

nix/script-tests/database-size-push.nix

@@ -0,0 +1,21 @@
+# `checks.script-test-database-size-push` — runs ../host-modules/lib/database-size-push.nix's
+# script against a symlinked directory, as a `DynamicUser` unit's state directory
+# is, with a stub `curl`. The cases are in ./database-size-push.sh.
+{
+  pkgs,
+  ...
+}:
+pkgs.runCommand "hyperhive-script-test-database-size-push"
+  {
+    nativeBuildInputs = [ pkgs.coreutils ];
+    SCRIPT_LINK = pkgs.writeText "database-size-push-link.sh" (
+      import ../host-modules/lib/database-size-push.nix { inherit (pkgs) lib; } {
+        dir = "@DIR@";
+        otlpPort = 8906;
+      }
+    );
+  }
+  ''
+    ${pkgs.bash}/bin/bash ${./database-size-push.sh}
+    touch "$out"
+  ''