Like the comment said, the template is pretty simple.

  {{- if .Values.server.sidecarContainers }}
  {{- toYaml .Values.server.sidecarContainers | nindent 8 }}
  {{- end }}

You can put a values.yaml with your sidecar snippet like below:

server:
  sidecarContainers:
  - name: testSideCar
    image: alpine

And deploy with helm install prom stable/prometheus -f values.yaml. Then you can find your sidecar section in server deployment yaml:

apiVersion: apps/v1
kind: Deployment
spec:
  template:
    spec:
      serviceAccountName: test-prometheus-server
      containers:
        - name: prometheus-server-configmap-reload
          image: "jimmidyson/configmap-reload:v0.3.0"

          <<... omitted ...>>

        - name: testSideCar
          image: alpine