benchmark-cli: add child tree support (#12021)

* benchmark-cli: add child tree support

* removed extra comments

* addressed pr comments

* clean up

* addressed pr comments

substrate/bin/node/cli/tests/benchmark_storage_works.rs

@@ -47,6 +47,7 @@ fn benchmark_storage(db: &str, base_path: &Path) -> ExitStatus {
 		.args(["--state-version", "1"])
 		.args(["--warmups", "0"])
 		.args(["--add", "100", "--mul", "1.2", "--metric", "p75"])
+		.arg("--include-child-trees")
 		.status()
 		.unwrap()
 }