[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!

Comienza comparando el editor de consultas de visualización de Grafana con la entrada de consulta de variables de Grafana sin restricciones descrita en el issue. Define el flujo de consulta completa, incluido COUNT NODE, y verifica que la visualización ya no fuerce una instrucción SELECT, manteniendo el comportamiento de consulta existente.

Escrito por el modelo de indexación a partir del texto del issue.