tests: runtime: cover Prometheus exporter output worker

Verify that the Prometheus exporter default creates and starts a single
output worker thread pool.

Signed-off-by: Stefano Tondo <stondo@gmail.com>

Author: stondo

tests/runtime/out_prometheus_exporter.c

@@ -113,6 +113,9 @@ static void test_http_server_options(void)
 
     out = flb_output_get_instance(ctx->config, out_ffd);
     TEST_CHECK(out != NULL);
+    TEST_CHECK(out->tp_workers == 1);
+    TEST_CHECK(out->is_threaded == FLB_TRUE);
+    TEST_CHECK(out->tp != NULL);
 
     prom_ctx = out->context;
     TEST_CHECK(prom_ctx != NULL);