[FLINK-4202] [metrics] Update restarting time metric documentation

This closes #2284.

docs/apis/metrics.md

@@ -409,10 +409,18 @@ Flink exposes the following system metrics:
         <td>currentLowWatermark</td>
         <td>The lowest watermark a task has received.</td>
       </tr>
+      <tr>
+        <td>lastCheckpointDuration</td>
+        <td>The time it took to complete the last checkpoint.</td>
+      </tr>
       <tr>
         <td>lastCheckpointSize</td>
         <td>The total size of the last checkpoint.</td>
       </tr>
+      <tr>
+        <td>restartingTime</td>
+        <td>The time it took to restart the job.</td>
+      </tr>
       <tr>
         <td>numBytesInLocal</td>
         <td>The total number of bytes this task has read from a local source.</td>

flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/ExecutionGraph.java

@@ -1306,8 +1306,10 @@ public class ExecutionGraph implements Serializable {
 
 	/**
 	 * Gauge which returns the last restarting time. Restarting time is the time between
-	 * JobStatus.RESTARTING and JobStatus.RUNNING. If it is still the initial job execution,
-	 * then the gauge will return 0.
+	 * JobStatus.RESTARTING and JobStatus.RUNNING or a terminal state if JobStatus.RUNNING was not
+	 * reached. If the job has not yet reached either of these states, then the time is measured
+	 * since reaching JobStatus.RESTARTING. If it is still the initial job execution, then the
+	 * gauge will return 0.
 	 */
 	private class RestartTimeGauge implements Gauge<Long> {