Add metrics scripts in preparation to move the builder to LUCI. (#22086)

* Add metrics scripts in preparation to move the builder to LUCI.

Bug:
  https://github.com/flutter/flutter/issues/67579

* set -ex to fail scripts inmediately.

testing/benchmark/generate_metrics.sh

+#!/bin/bash
+#
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+#
+# This script is expected to run from $ENGINE_PATH/src/out/host_release/
+# it is currently used only by automation to collect and upload metrics.
+
+set -ex
+
+./txt_benchmarks --benchmark_format=json > txt_benchmarks.json
+./fml_benchmarks --benchmark_format=json > fml_benchmarks.json
+./shell_benchmarks --benchmark_format=json > shell_benchmarks.json
+./ui_benchmarks --benchmark_format=json > ui_benchmarks.json
+

testing/benchmark/upload_metrics.sh

+#!/bin/bash
+#
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+#
+# This script is expected to run from $ENGINE_PATH/src/flutter/testing/benchmark
+# it is currently used only by automation to collect and upload metrics.
+
+set -ex
+
+pub get
+dart bin/parse_and_send.dart ../../../out/host_release/txt_benchmarks.json
+dart bin/parse_and_send.dart ../../../out/host_release/fml_benchmarks.json
+dart bin/parse_and_send.dart ../../../out/host_release/shell_benchmarks.json
+dart bin/parse_and_send.dart ../../../out/host_release/ui_benchmarks.json