def list_service(service_name = nil, section_name = nil, graph_name = nil)
      graphs = list_graph(service_name, section_name, graph_name)
      services = {}
      graphs.each do |graph|
        service_name = graph['service_name']
        services[service_name] ||= true
      end
      services.keys
    end
def list_service(service_name = nil, section_name = nil, graph_name = nil)
      graphs = list_graph(service_name, section_name, graph_name)
      services = {}
      graphs.each do |graph|
        service_name = graph['service_name']
        services[service_name] ||= true
      end
      services.keys
    end
A Helper: Get the list of services
 @return [Array] list of services
 @example
 [
   "service_name1",
   "service_name2",
 ]