Provide a `isSet` template function for field existence
Provide a `isSet` template function for field existence

Example of use:

  var template = `{{ if isSet .Object "Data" }}{{ .Object.Data }}{{else}}No Data in Object{{end}}``

Signed-off-by: Alban Dericbourg <df3756abd1bd97927cfd0e8a4ae9e93dc6a9eb8d@criteo.com>

Thib17/graphite-remote-adapter,Thib17/graphite-remote-adapter,Thib17/graphite-remote-adapter