metrics: Add next gen grafana json files (#61688)

ref #60864

Author: yibin87

pkg/metrics/nextgengrafana/README.md

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+## About
+
+Use [jsonnet](https://github.com/google/go-jsonnet) to generate Grafana-compliant json scripts for use with TiDB.
+
+Why jsonnet?
+
+1. jsonnet is a DSL created by Google for json, which is good for advanced json editing work.
+2. Grafana provides the [jsonnet library](https://grafana.github.io/grafonnet-lib/) specifically for generating Grafana json, which makes maintaining TiDB's json scripts much easier.
+
+## Usage
+
+1. Modify the jsonnet files (e.g. tidb_summary.jsonnet).
+2. Run `generate_json.sh` to generate the json files by the jsonnet files.
+3. Commit the modifications.

pkg/metrics/nextgengrafana/generate_json.sh

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+#!/usr/bin/env bash
+# Copyright 2025 PingCAP, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+go install github.com/google/go-jsonnet/cmd/jsonnet@latest
+
+# ref https://github.com/grafana/grafonnet-lib/issues/338, use the forked 
+# repo which implements the addOverride and addTransformation funtion for 
+# new table.
+# TODO: update to https://github.com/grafana/grafonnet
+git clone https://github.com/nolouch/grafonnet-lib.git
+
+export JSONNET_PATH=grafonnet-lib
+jsonnet tidb_summary.jsonnet > tidb_summary.json
+jsonnet tidb_summary_with_keyspace_name.jsonnet > tidb_summary_with_keyspace_name.json
+jsonnet tidb_resource_control_with_keyspace_name.jsonnet > tidb_resource_control_with_keyspace_name.json
+rm -rf $JSONNET_PATH
+