[devtools] Add a user metric for per block coverage

DevTools will soon support block coverage again, and this
metric tracks how often it gets used.

Bug: chromium:1004203
Change-Id: I002d48361d5c8b5a130b3d444f89e8b7edae3d5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891295Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711231}

tools/metrics/histograms/enums.xml

@@ -13855,6 +13855,7 @@ Called by update_net_error_codes.py.-->
   <int value="32" label="Audits' view trace clicked"/>
   <int value="33" label="Network panel film strip started recording"/>
   <int value="34" label="Coverage report was filtered"/>
+  <int value="35" label="Coverage started (per block)"/>
 </enum>
 
 <enum name="DevToolsBackgroundService">