es: Logstash_Prefix_Key with static prefix

## Bug Report

**Describe the bug**

Specifying the `Logstash_Prefix_Key` option with a static prefix (like `containers-$kubernetes['namespace_name']`) doesn't work, although they should be valid according to [docs](https://docs.fluentbit.io/manual/administration/configuring-fluent-bit/classic-mode/record-accessor#limitations-of-record_accessor-templating).

**To Reproduce**
- Rubular link if applicable:
- Example log message if applicable:
```
[2024/04/29 14:11:31] [error] [record accessor] syntax error, unexpected '$', expecting end of file at '$containers-$kubernetes['namespace_name']'
[2024/04/29 14:11:31] [error] [output:es:es.0] invalid logstash_prefix_key pattern '(null)'
[2024/04/29 14:11:31] [error] [output:es:es.0] cannot initialize plugin
[2024/04/29 14:11:31] [error] [output] failed to initialize 'es' plugin
[2024/04/29 14:11:31] [error] [engine] output initialization failed
```
- Steps to reproduce the problem: See in description above.

**Expected behavior**

The log is inserted into the index "containers-default" if the log came from a container in the namespace "default" and the kubernetes filter plugin added the kubernetes.namespace_name

**Screenshots**

**Your Environment**

* Version used:
* Configuration:
* Environment name and version (e.g. Kubernetes? What version?):
* Server type and version:
* Operating System and version:
* Filters and plugins:

**Additional context**

This is the relevant code line, which only checks if the first character of the Logstash_Prefix_Key value is `$` and prepends `$` otherwise, while (IMO) it should check if the value *contains* a `$`:
https://github.com/fluent/fluent-bit/blob/9c7a4bf20a6949eaa2f60f1d6635707d0e2e6f13/plugins/out_es/es_conf.c#L329