test: negative pgrst_db_pool_available in metrics

Proves the failure on https://github.com/PostgREST/postgrest/issues/4622.

This doesn't require additional test infra, only nginx. Taking advantage
of the `stream {}` context which is also compatible with unix socket
besides TCP.

test/io/nginx/nginx.conf

@@ -0,0 +1,13 @@
+# the PG* variables are replaced by preprocessing, not done by nginx itself
+daemon off;
+pid        ./nginx.pid;
+
+events {}
+
+stream {
+  server {
+    listen unix:$PGPROXYHOST/.s.PGSQL.5432;
+    proxy_timeout $PGPROXY_TIMEOUT;
+    proxy_pass unix:$PGHOST/.s.PGSQL.5432;
+  }
+}