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/host-modules/lib/database-size-push.nix

@@ -0,0 +1,37 @@
+# The hive-matrix container's database-size step: measures a directory and
+# pushes the byte count as the gauge `hyperhive.matrix.database_bytes` to the
+# forwarder's OTLP/HTTP metrics receiver on loopback.
+#
+# `dir` is a `DynamicUser` unit's `/var/lib/<name>`, which systemd makes a
+# symlink to `/var/lib/private/<name>`. `--dereference-args` measures the
+# directory behind the link and works unchanged when `dir` is a real
+# directory; without it `du` reports the link itself (a few bytes).
+#
+# A result under 4096 bytes is not a database, since a directory entry alone
+# is that big: the step warns on stderr and pushes nothing, so the gauge
+# stays absent rather than wrong.
+#
+# Pure function — NOT a NixOS module. Needs `du`, `cut`, `date` and `curl`.
+{ lib }:
+{ dir, otlpPort }:
+''
+  set -euo pipefail
+
+  bytes="$(du --summarize --bytes --dereference-args ${lib.escapeShellArg dir} | cut -f1)"
+  case "$bytes" in
+    "" | *[!0-9]*)
+      echo "du answered a non-number: bytes='$bytes'" >&2
+      exit 1
+      ;;
+  esac
+  if [ "$bytes" -lt 4096 ]; then
+    echo "warning: ${dir} measures $bytes bytes, too small for a database; pushing nothing" >&2
+    exit 0
+  fi
+
+  printf '{"resourceMetrics":[{"resource":{"attributes":[{"key":"service.name","value":{"stringValue":"hive-matrix"}}]},"scopeMetrics":[{"metrics":[{"name":"hyperhive.matrix.database_bytes","gauge":{"dataPoints":[{"asInt":"%s","timeUnixNano":"%s"}]}}]}]}]}' \
+    "$bytes" "$(date +%s%N)" |
+    curl --fail --silent --show-error --max-time 20 \
+      -H 'Content-Type: application/json' --data-binary @- \
+      http://127.0.0.1:${toString otlpPort}/v1/metrics
+''