Make buffer volume metrics PrometheusRules configurable

[sebastiangaiser]

Is your feature request related to a problem? Please describe.
In #1790 PrometheusRule overrides were introduced. Unfortunately only for FluentD/Fluent-bit metrics and not for buffer volume metrics.

Describe the solution you'd like
Please add the same overrides for the buffer volume metrics, e.g.:

fluentd:
  bufferVolumeMetrics:
    prometheusRules: true
    prometheusRulesOverride:
    - alert: FluentbitBufferSize
      labels:
         severity:  warning # former 'critical'
fluentd:
  bufferVolumeMetrics:
    prometheusRules: true
    prometheusRulesOverride:
    - alert: FluentdPredictedBufferGrowth
      expr: ...
syslogng:
  bufferVolumeMetrics:
    prometheusRules: true
    prometheusRulesOverride:
    ...

Describe alternatives you've considered
Disable prometheusRules and deploy them separately.

Additional context

[sebastiangaiser]

This may also solve #1923