cryptohome: add FreeDiskSpaceTotalFreedInMb

Add a metric to measure how much space is being cleared by the
automatic disk cleanup.

Used by crrev.com/c/2007128

BUG=chromium:1043037
TEST=manual

Change-Id: I00e88ca84466fa5b3032957641b8b5512649c3e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2005991Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Victor-Gabriel Savu <vsavu@google.com>
Cr-Commit-Position: refs/heads/master@{#734379}

tools/metrics/histograms/histograms.xml

@@ -30110,6 +30110,17 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Cryptohome.FreeDiskSpaceTotalFreedInMb" units="MiB"
+    expires_after="M87">
+  <owner>vsavu@chromium.org</owner>
+  <owner>weifangsun@chromium.org</owner>
+  <summary>
+    In low disk space scenarios, Cryptohome performs a lot of disk space
+    operations to erase data. This is the amount of space that was released by
+    the automatic cleanup.
+  </summary>
+</histogram>
+
 <histogram name="Cryptohome.FreeDiskSpaceTotalTime" units="ms"
     expires_after="M81">
   <owner>slangley@chromium.org</owner>