[Feature request] Add COUNT NODE query on grafana visualization

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/iotdb/issues) and found nothing similar.

### Motivation

On grafana variables you have the choice to write an entire query without limitations has shown below:

(i use this query to count the number of cores of a virtual machine, each of which i take data from)

i can't use this syntax on the grafana visualization because i'm forced to use a SELECT statement:


### Solution

Add another drop-down selection with a full-customized query editior that allows you to use all the language provided queries as in the grafana variables

### Alternatives

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

まず、Issue で説明されている制限のない Grafana 変数クエリ入力と、Grafana の可視化クエリエディターを比較します。COUNT NODE を含むフルクエリのフローを定義し、既存のクエリ動作を維持したまま、可視化で SELECT 文が強制されなくなっていることを確認します。

索引モデルが issue の本文から書いたものです。