To be specific, here is the permalink to the relevant line of code: https://github.com/istio/istio/blob/e3a376610c2f28aef40296aac722c587629123c1/install/kubernetes/helm/istio/templates/sidecar-injector-configmap.yaml#L84
{{ "[[ .ProxyConfig.ZipkinAddress ]]" }}
The [[ and ]] seems alien to me, and in the helm chart developer guide doc here, it doesn't show any example or documentation about [[ and ]] syntax.
Also, when I tried to render my istio installation (using helm template command), the {{ "[[ .ProxyConfig.ZipkinAddress ]]" }} part only rendered as [[ .ProxyConfig.ZipkinAddress ]]. So I guess that [[ and ]] is not part of helm template syntax. My guess it would be internal istio's related syntax, which I don't know what exactly it is.
Any idea?
I got the answer after posting the same question on the Istio's google group here. Without discrediting the author who answering me in the google group, the answer is yes it is a template of a template. The template syntax is used by sidecar injection described in here: https://istio.io/docs/setup/kubernetes/sidecar-injection/#template