fix(admin): metrics endpoint not responding with Content-Type header

The prometheus metrics text format requires `Content-Type` header
for correct scraping which fails otherwise. Closes #4271.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

test/io/test_io.py

@@ -1730,6 +1730,7 @@ def test_admin_metrics(defaultenv):
     with run(env=defaultenv, port=freeport()) as postgrest:
         response = postgrest.admin.get("/metrics")
         assert response.status_code == 200
+        assert response.headers["Content-Type"] == "text/plain; charset=utf-8"
         assert "pgrst_schema_cache_query_time_seconds" in response.text
         assert 'pgrst_schema_cache_loads_total{status="SUCCESS"}' in response.text
         assert "pgrst_db_pool_max" in response.text