Allow ExceptionHooks to format exception causes

The new method allows implementors to group platform-specific exceptions
that have the same cause into one metric bucket.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Ie9e5db43c6b3444e84c46bf7fc3da0db15c9ebf9
diff --git a/Documentation/dev-plugins.txt b/Documentation/dev-plugins.txt
index a91a138..223ec71 100644
--- a/Documentation/dev-plugins.txt
+++ b/Documentation/dev-plugins.txt
@@ -2752,6 +2752,9 @@
 this interface can be used to retry the request instead of failing it
 immediately.
 
+It also allows implementors to group exceptions that have the same
+cause into one metric bucket.
+
 [[mail-soy-template-provider]]
 == MailSoyTemplateProvider