Update metrics one pager

Add instructions for how to use shorthand to set variant's summary
attribute.

Change-Id: I5233dadd21c60f5c354e70d814f613e5f1f8c27c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388911
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803943}

tools/metrics/histograms/one-pager.md

@@ -67,7 +67,7 @@ If we define the following:
     <variant name=".ExtensionApp" summary="the ExtensionApp">
       <owner>you@chromium.org</owner>
     </variant>
-    <variant name=".HistoryContents" summary=" the HistoryContents"/>
+    <variant name=".HistoryContents" summary="the HistoryContents"/>
   </token>
 </histogram>
 ```
@@ -119,6 +119,27 @@ MyHist2.V1
 MyHist2.V2
 ```
 
+If you omit the summary attribute in the `<variant>`'s tag, We'll set it to be
+the same as its name. Example:
+
+```
+<histogram name="MyHist1.{MyVariants}">
+  <summary>MyHist1 with {MyVariants}.</summary>
+  <token key="MyVariants">
+      <variant name="Red"/>
+      <variant name="Green"/>
+    </token>
+  </token>
+</histogram>
+```
+
+The complete list of histograms and their summary will be:
+
+```
+MyHist1.Red: MyHist1 with Red
+MyHist1.Green: MyHist1 with Green
+```
+
 The variant name is allowed to be an empty string, to make it easy to define
 patterns that include legacy histogram names. When defining new histograms,
 prefer an **explicit variant name** instead. Example: