fix FinishedMount::populate() to log backing_store_type

Summary: This field was added to `FinishedMount` but never added to `populate()`. This diff fixes this so we log the `backing_store_type` to scuba. The motivation behind sending this diff was that I tried to use this data before (to see the distribution of types) and it was missing.

Reviewed By: jdelliot

Differential Revision: D67767064

fbshipit-source-id: 6cc9e4d3872f0d2adf04f15f8edf1c9aac078264

eden/fs/telemetry/LogEvent.h

@@ -294,6 +294,7 @@ struct FinishedMount : public EdenFSEvent {
         inode_catalog_type(inode_catalog_type) {}
 
   void populate(DynamicEvent& event) const override {
+    event.addString("backing_store_type", backing_store_type);
     event.addString("repo_type", repo_type);
     event.addString("repo_source", repo_source);
     event.addString("fs_channel_type", fs_channel_type);