chore: adding Grafana config for cluster monitor

[shuiyisong]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

This pr mainly adds a new Grafana config json for cluster monitoring. The result should look like following

Checkout grafana/README.md for more detail.

Minimal reproduce steps

1. start a GreptimeDB cluster on local machine (including a running etcd)
2. start a Prometheus instance. Config file are shown below.
3. start a Grafana instance.
4. log in to Grafana and config Prometheus data source using http://localhost:9090 (default address for Prometheus)
5. create a new dashboard and import grafana/greptimedb-cluster.json

Prometheus config file

global:
  scrape_interval: 5s
  evaluation_interval: 5s

scrape_configs:
  - job_name: metasrv
    static_configs:
    - targets: ['<host>:<ip>'] # replace with metasrv's host and ip
      labels:
        greptime_pod: metasrv

  - job_name: datanode
    static_configs:
    - targets: ['<host>:<ip>'] # replace with datanode-1's host and ip
      labels:
        greptime_pod: datanode1
    - targets: ['<host>:<ip>'] # replace with datanode-2's host and ip
      labels:
        greptime_pod: datanode2
    - targets: ['<host>:<ip>'] # replace with datanode-3's host and ip
      labels:
        greptime_pod: datanode3

  - job_name: frontend
    static_configs:
    - targets: ['<host>:<ip>'] # replace with frontend's host and ip
      labels:
        greptime_pod: frontend

  - job_name: prometheus
    static_configs:
      - targets: ['localhost:9090']

# this config is merely for writing data into GreptimeDB to trigger metrics changing
remote_write:
  - url: http://localhost:4904/v1/prometheus/write

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[evenyag]

Me: Looks...... 🫢 Grafana: Good to me

[tisonkun]

Thank you!