Frequent schema changes due to freeStorageSize limit FTDC retention

In SERVER-42037 we introduced a freeStorageSize metric that reports the amount of bytes available for reuse in the oplog table. But this value is only appended to serverStatus if it is non-zero, which creates frequent schema changes in the case where the oplog table has only a small amount of free space. In one test back-of-the-envelope calculation indicates this was cutting FTDC retention in half. The value should be included in serverStatus even if it's 0.