If you have key request, you can create custom chart with this measure:
then you can filter out requests by it's name if you want particular one. Then you will have what you need. you have as well measures for requests without 5xx errors with errors (all types) without etc.
there is metric for failure rate in general is available there as well. Go to create custom chart => pick Services Category and on metrics list you can write failure key request. You will see bunch of available metrics. Check them there. If you wont have count not percentage, you will have to go to servcies that is the most on the front (for example nginx etc) and you can create multidimensional analysis there which will be able to count number of requests per response code.