Update 'Metric' reference documentation. (#3613)

Co-authored-by: kkraune <[email protected]>

en/reference/unit-metrics-reference.html

@@ -17,6 +17,10 @@
       <td>bucket</td>
       <td>A chunk of documents managed by a distributor service</td>
     </tr>
+    <tr>
+      <td>buffer</td>
+      <td>A buffer</td>
+    </tr>
     <tr>
       <td>byte</td>
       <td>A collection of 8 bits</td>
@@ -25,10 +29,18 @@
       <td>byte/second</td>
       <td>A unit of storage capable of holding 8 bits</td>
     </tr>
+    <tr>
+      <td>class</td>
+      <td>A instance of a Java class</td>
+    </tr>
     <tr>
       <td>connection</td>
       <td>A link used for communication between a client and a server</td>
     </tr>
+    <tr>
+      <td>context switch</td>
+      <td>A context switch</td>
+    </tr>
     <tr>
       <td>deployment</td>
       <td>A deployment on hosted Vespa</td>