I was not able to figure out how to make the directive work properly by setting headerTooltips as strings. Directive developers do it using a different implementation than yours, which can be seen in this Plunker .
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upareneStavkePromise.then(function(upareneStavkeData){
$log.debug(upareneStavkeData);
$scope.ucitaniUpareniPodaci = true;
$scope.gridOptionsUpareniPodaci.data = upareneStavkeData.grupe;
upareneStavkeTotals = upareneStavkeData.totals;
var headerClass = ".ui-grid-header-cell-primary-focus";
var colDefs = $scope.gridOptionsUpareniPodaci.columnDefs;
var headers = document.querySelectorAll(headerClass);
angular.forEach(headers,function(value,key){
headers[key].title = colDefs[key].headerTooltip;
});
});