Add histogram expiration description.

Bug: 744672
Change-Id: I738348739de70fb0fc4273bbe73a936b98520361
Reviewed-on: https://chromium-review.googlesource.com/1005791
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550117}

tools/metrics/histograms/histograms.xml

@@ -20,7 +20,7 @@ Take a moment to page through the contents of this file. You will find
 two sections:
 
 * The histograms section describes base histograms, giving their name,
-  and optionally their units or enum type, a short one-line summary,
+  and optionally their units or enum type, expiration, a short one-line summary,
   and optionally a more detailed description.
 * The histogram_suffixes section provides a compact way of defining histograms
   by applying affixes to existing histograms. This could be done recursively
@@ -29,6 +29,18 @@ two sections:
 
 Enum types of histograms are defined and described in enums.xml.
 
+Histogram expiration can be specified as a date in YYYY-MM-DD format or as a
+Chrome milestone. For example:
+  <histogram name="HistogramName" expires_after="2018-01-01">
+or
+  <histogram name="HistogramName" expires_after="M68">
+
+After a histogram expires, it will not be recorded (nor uploaded to the UMA
+servers). The code to record it becomes dead code, and should be removed from
+the codebase along with marking the histogram definition as obsolete. However,
+if histogram would remain useful, the expiration should be extended
+accordingly.
+
 Each histogram_suffixes tag lists the histograms that it affects. The complete
 list of histograms is computed by appending (or prepending - see below) the
 histogram_suffixes suffix names to each of the affected histograms. For example,