metrics: fix "values" to be count not bytes, to match prev behavior

https://github.com/NixOS/nixpkgs/pull/36896#discussion_r174163744

pkgs/top-level/metrics.nix

@@ -35,7 +35,7 @@ runCommand "nixpkgs-metrics"
       [[ -n $x ]] || exit 1
       echo "$name.allocations $x B" >> $out/nix-support/hydra-metrics
 
-      x=$(sed -e 's/.*values allocated bytes: \([0-9]\+\).*/\1/ ; t ; d' stats)
+      x=$(sed -e 's/.*values allocated count: \([0-9]\+\).*/\1/ ; t ; d' stats)
       [[ -n $x ]] || exit 1
       echo "$name.values $x" >> $out/nix-support/hydra-metrics
     }