[Feat] add support for creating ServiceMonitor in Helm chart

[Hexoplon]

Add support for creating a ServiceMonitor directly in the helm chart. This simplifies configuration of monitoring for the stack

Implements #267

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[YuhanLiu11]

hey @Hexoplon thanks for submitting the PR! Can you provide more instruction on how to use this new helm template?

[Hexoplon]

@YuhanLiu11 Thanks for the feedback! I updated the observability docs to enable the serviceMonitor install in the helm release values, instead of including the additionalServiceMonitor field in the kube-prom-stack values.