Add 5xx example Athena query

Add an example AWS Athena query to give the total number of 5xx responses,
the total number of requests, and the percentage of 5xx responses of the
total number of requests.

In cases where we have high 5xx responses, it saves you having to work
out the total and % manually.

source/manual/query-cdn-logs.html.md

@@ -102,6 +102,22 @@ GROUP BY url
 ORDER BY count DESC
 ```
 
+### Total number of 5xx's responses, total requests and percentage of 5xx's were served to users in a timeframe
+
+```sql
+SELECT SUM(responses) as "Total 5xx Responses", SUM(requests) "Total Requests",
+    SUM(responses) * 100.0 / SUM(SUM(requests)) OVER () AS "5xx % of Total Requests"
+FROM (
+    SELECT
+        COUNT(CASE WHEN status/100=5 THEN 1 ELSE NULL END) as responses,
+        COUNT(*) AS requests
+    FROM fastly_logs.govuk_www
+    WHERE date = 2 AND month = 6 AND year = 2021
+    AND request_received >= TIMESTAMP '2021-06-02 15:00'
+    AND request_received < TIMESTAMP '2021-06-02 17:00'
+)
+```
+
 ### Find out the number of requests per status per hour for a GOV.UK URL
 
 ```sql