[Proposal] provide out-of-box grafana dashboards

## Why you need it?

Though we already have metrics service and prometheus exporter, there is still one more work to do in terms of observability, i.e. a visualizer. Nowadays, users have to maintain a self-made grafana panel to observe Seata metrics.

## How it could be?

I propose to implement an out-of-box grafana panel for users to quickly get hands on. And we can integrate the grafana panel to Seata console, to make Seata console a one-stop control plane for Seata transaction.

The grafana panel should be like something in this blog [post](https://seata.io/zh-cn/blog/seata-observable-practice).

## Other related information

How do you guys think?

Start with the existing metrics service, Prometheus exporter, and Seata console entry points; the issue does not name repository files or tests. Compare the proposed dashboards with the linked observability blog, then define the dashboard scope and console integration before implementation. Done means users can obtain Grafana dashboards out of the box and access them through the console.

Written by the indexing model from the issue text.