[FLINK-7626] [docs][metrics] Add some checkpoints metric description …

…in docs

This closes apache#4677.

docs/monitoring/metrics.md

@@ -808,7 +808,7 @@ Thus, in order to infer the metric identifier:
  </thead>
  <tbody>
  <tr>
-  <th rowspan="3"><strong>Job (only available on JobManager)</strong></th>
+  <th rowspan="9"><strong>Job (only available on JobManager)</strong></th>
   <td>lastCheckpointDuration</td>
   <td>The time it took to complete the last checkpoint.</td>
  </tr>
@@ -820,6 +820,30 @@ Thus, in order to infer the metric identifier:
   <td>lastCheckpointExternalPath</td>
   <td>The path where the last checkpoint was stored.</td>
  </tr>
+ <tr>
+  <td>lastCheckpointRestoreTimestamp</td>
+  <td>Timestamp when the last checkpoint was restored at the coordinator.</td>
+ </tr>
+ <tr>
+  <td>lastCheckpointAlignmentBuffered</td>
+  <td>The number of buffered bytes during alignment over all subtasks for the last checkpoint.</td>
+ </tr>
+ <tr>
+  <td>numberOfInProgressCheckpoints</td>
+  <td>The number of in progress checkpoints.</td>
+ </tr>
+ <tr>
+  <td>numberOfCompletedCheckpoints</td>
+  <td>The number of successfully completed checkpoints.</td>
+ </tr> 
+ <tr>
+  <td>numberOfFailedCheckpoints</td>
+  <td>The number of failed checkpoints.</td>
+ </tr>
+ <tr>
+  <td>totalNumberOfCheckpoints</td>
+  <td>The number of total checkpoints (in progress, completed, failed).</td>
+ </tr>
  <tr>
   <th rowspan="1">Task</th>
   <td>checkpointAlignmentTime</td>