Add continuous query for ETag caching events

queries/events/etag_caching_per_endpoint_per_hour.txt

+CREATE CONTINUOUS QUERY etag_caching_per_endpoint_per_hour ON $DATABASE
+BEGIN
+    SELECT sum(count) AS "count"
+    INTO year.etag_caching_per_endpoint_per_hour
+    FROM "$DEFAULT_RETENTION_POLICY".events
+    WHERE "event" =~ /^etag_caching_/
+    GROUP BY time(1h), event, endpoint
+END