fix(tvix/castore/src): record rq.handle field in read()

This makes it easier to separate concurrent requests on the same inode.

Change-Id: I7637c1d889336beeb0d186182ce22fbf60fd16c3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11447
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/castore/src/fs/mod.rs

@@ -713,7 +713,7 @@ where
         Ok(())
     }
 
-    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.offset = offset, rq.size = size), parent = self.file_handles.read().get(&handle).and_then(|x| x.0.id()))]
+    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.handle = handle, rq.offset = offset, rq.size = size), parent = self.file_handles.read().get(&handle).and_then(|x| x.0.id()))]
     fn read(
         &self,
         _ctx: &Context,