ZOOKEEPER-4703: Datadir and DatalogDir size are incorrect

[SiyaoIsHiding]

As in the class FileTxnSnapLog

    //the directory containing
    //the transaction logs
    final File dataDir;
    //the directory containing
    //the snapshot directory
    final File snapDir;

The DataDir is the one for logs. Therefore, the file path for DataDirSize and LogDirSize should be swapped.

[maoling]

Great catch. Incredible. this issue existed for almost 9 years.
Since it only exposed some metrics(e.g. /commands/dirs) and don't affect the core function, so nobody realized it.

[maoling]

@SiyaoIsHiding
Thanks for your contribution.

[anmolnar]

@SiyaoIsHiding @maoling

Great fix, but where's the unit test that catches it?
Please also make sure it's backported to active branches.

[li4wang]

#2043 has unit test for it.