Merge pull request #172 from prometheus/revert-166-patch-1

Revert #166 and add explanation why the formula is in fact correct.

content/docs/practices/histograms.md

@@ -76,8 +76,12 @@ following expression yields the Apdex score for each job over the last
     (
       sum(rate(http_request_duration_seconds_bucket{le="0.3"}[5m])) by (job)
     +
-      sum(rate(http_request_duration_seconds_bucket{le="1.2"}[5m])) by (job) / 2
-    ) / sum(rate(http_request_duration_seconds_count[5m])) by (job)
+      sum(rate(http_request_duration_seconds_bucket{le="1.2"}[5m])) by (job)
+    ) / 2 / sum(rate(http_request_duration_seconds_count[5m])) by (job)
+
+Note that we divide the sum of both buckets. The reason is that the histogram
+buckets are cumulative. The `le="0.3"` bucket is also contained in the `le="1.2"`
+bucket; dividing it by 2 corrects for that.
 
 ## Quantiles