getCodeCoverageJSON.cls

String baseURL =  'https://' + System.URL.getSalesforceBaseUrl().getHost();

string queryStr = 'SELECT+NumLinesCovered,ApexClassOrTriggerId,ApexClassOrTrigger.Name,NumLinesUncovered,Coverage+FROM+ApexCodeCoverageAggregate';

String ENDPOINT = baseURL + '/services/data/v40.0/tooling/';

HttpRequest req = new HttpRequest();

req.setEndpoint(ENDPOINT + 'query/?q=' + queryStr);
req.setHeader('Authorization', 'Bearer ' + UserInfo.getSessionID());
req.setHeader('Content-Type', 'application/json');
req.setMethod('GET');
req.setTimeout(80000);

Http http = new Http();
HTTPResponse res = http.send(req);

System.debug(res.getBody());