KAFKA-5738; Upgrade note for cumulative count metric (KIP-187)

Author: Rajini Sivaram <rajinisivaram@googlemail.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #4014 from rajinisivaram/MINOR-upgrade-KIP-187

docs/upgrade.html

@@ -76,6 +76,9 @@
         will return <code>0.0</code> in such cases to minimise the probability of breaking users who read the value of every client
         metric (via a <code>MetricsReporter</code> implementation or by calling the <code>metrics()</code> method).
         <code>org.apache.kafka.common.Metric#metricValue()</code> can be used to retrieve numeric and non-numeric metric values.</li>
+    <li>Every Kafka rate metric now has a corresponding cumulative count metric with the suffix <code>-total</code>
+        to simplify downstream processing. For example, <code>records-consumed-rate</code> has a corresponding
+        metric named <code>records-consumed-total</code>.</li>
     <li>Mx4j will only be enabled if the system property <code>kafka_mx4jenable</code> is set to <code>true</code>. Due to a logic
         inversion bug, it was previously enabled by default and disabled if <code>kafka_mx4jenable</code> was set to <code>true</code>.</li>
 </ul>