feat: Add ServiceMonitor for data-prepper

[darnaut]

Description

Adds ServiceMonitor resource for monitoring with Prometheus.

Issues Resolved

N/A

Check List

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For any changes to files within Helm chart directories:

• Helm chart version bumped
• Helm chart CHANGELOG.md updated to reflect change

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[peterzhuamazon]

Hi @TheAlgo @prudhvigodithi could you take a look on this?

Thanks.

[sgorshkov85]

Hello.
Currently(with what to have: https://github.com/opensearch-project/helm-charts/blob/data-prepper-0.1.0/charts/data-prepper/templates/deployment.yaml#L49-L57), it makes sense to use PodMonitor instead.
OR - adjust https://github.com/opensearch-project/helm-charts/blob/data-prepper-0.1.0/charts/data-prepper/templates/service.yaml#L9-L15 to add server(port 4900) to the list, so the approach with service monitor will also work.

[darnaut]

@sgorshkov85 For context, the choices made here were intentional:

1. ServiceMonitor is typically the recommend approach by Prometheus and also works better here given that there's a service available and it will only match the endpoints available in the service (e.g. it won't scrape pods removed from the service)
2. The port is not enabled by default because the ServiceMonitor itself is not enabled by default. I'm avoiding exposing by default a port which wasn't previously exposed. It's left up to the Helm chart user to enable the required port when enabling the ServiceMonitor.

[sgorshkov85]

@darnaut I got your point about ServiceMonitor, it was my first option as well.
I may lose something, but I don't see how it should work with these changes.

1. To use ServiceMonitor monitor we need to configure the service resource. It could be done by defining the port variable:

    ports:
 - name: server
   port: 4900

2. It will cause an error during the creation deployment resource, because of setting the same port number two times:

        ports:
        {{- range .Values.ports }}
          - name: {{ .name }}
            containerPort: {{ .port }}
            protocol: TCP
        {{- end }}
          - name: server
            containerPort: {{ (.Values.config).serverPort | default 4900 }}
            protocol: TCP

In the deployment this port is hardcoded, because it is used for the readiness and liveness probes.

[darnaut]

@sgorshkov85 if there's some issue for you, please submit a PR fixing the duplicate port issue. this would be a problem regardless of this change (e.g. can't expose the server port in the service).