Alternator: missing ItemCount field in return of DescribeTable

[nyh]

The return from DescribeTable is missing the ItemCount field. According to DynamoDB documentation, ItemCount should have:

The number of items in the specified table. DynamoDB updates this value approximately every six hours. Recent changes might not be reflected in this value.

In addition to base tables, indexes should also have this ItemCount estimate.

Unfortunately, there is no obvious way to implement this feature efficiently in Scylla. We have a probabilistic mechanism for estimating the number of partitions in a set of sstables, but ItemCount is about individual rows, not partitions. Moreover, whatever estimate we get includes just the data on one node - we'll probably need to collect estimates from all nodes - and when (periodically? during the request? what happens when one of the nodes is down)?
Moreover it's not clear to me (we'll need to find an answer to this) whether a probabilistic, approximate count, is good enough - or if this must be an accurate count of items (albeit only true to six hours ago).

We have an xfailing tests for this feature: test_describe_table.py::test_describe_table_item_count, test_gsi.py::test_gsi_describe and test_lsi.py::test_lsi_describe_fields.