histograms: Add histogram for stateful partition used space

Add UMA histogram "Platform.StatefulUsedSpace" for Chrome OS.
    This tracks the space used for the stateful partition in GB.

Implementation CL:
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1775464

Bug: chromium:999309
Test: ran validate_format.py
Change-Id: If2638edf7b48f8420c1893b6cdc3f71623f5ae17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776853
Commit-Queue: Alexis Savery <asavery@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696624}

tools/metrics/histograms/histograms.xml

@@ -104117,6 +104117,15 @@ uploading your change for review.
   <summary>Chrome OS stateful partition usage level.</summary>
 </histogram>
 
+<histogram name="Platform.StatefulUsedSpace" units="GB"
+    expires_after="2030-08-29">
+  <owner>asavery@chromium.org</owner>
+  <owner>gwendal@chromium.org</owner>
+  <summary>
+    Chrome OS stateful partition space used. Sampled once daily.
+  </summary>
+</histogram>
+
 <histogram name="Platform.Storage.Flash.BadBlocks">
   <owner>dehrenberg@chromium.org</owner>
   <owner>cast-analytics@google.com</owner>