It's always better to first check the RESTful API documentations. In your case, you can get those values either by applying regexp to XML contents, or read/decode it in JSON format:
key = 'Q.N.DE.W2.S11.S13.N.A.LE.F4.T._Z.XDC._T.S.V.N._T';
api = 'https://sdw-wsrest.ecb.europa.eu/service/';
url = [api 'data/QSA/' key];
raw = webread(url);
data = jsondecode(raw);
% get ids/values
ObsDimension = {data.structure.dimensions.observation.values.id}.';
ObsValue = struct2table(data.dataSets.series.x0_0_0_0_0_0_0_0_0_0_0_0_0_0_0_0_0_0.observations); % ObsValue are stored here
% create a new table from these 2 arrays
res = table(ObsDimension, ObsValue{1, :}.', 'VariableNames', {'ObsDimension', 'ObsValue'})
You could also use regexp:
key = 'Q.N.DE.W2.S11.S13.N.A.LE.F4.T._Z.XDC._T.S.V.N._T';
api = 'https://sdw-wsrest.ecb.europa.eu/service/';
url = [api 'data/QSA/' key];
opts = weboptions('ContentType', 'text');
raw = webread(url, opts);
ObsDimensionNew = regexp(raw, '(?<=<generic:ObsDimension value=")(.*?)[^"/>]*', 'match').';
ObsValueNew = regexp(raw, '(?<=<generic:ObsValue value=")(.*?)[^"/>]*', 'match').';
resNew = table(ObsDimensionNew, ObsValueNew, 'VariableNames', {'ObsDimension', 'ObsValue'})
