Added Array.info_complete

[TomAugspurger]

Now that Store.getsize is a thing, we can do info_complete which includes the number of chunks written and the size of those bytes.

The current implementation unfortunately does two list_prefixes on the same prefix. The first to get the count of chunks initialized and the second to get the bytes stored under a prefix. Unfortunately, these down compose well. I haven't thought of a nice way to eliminate that yet. We can do this naively by doing a single list_prefix and then counting the number of keys as we call getsize on each. But we also have a Store.getsize_prefix for those stores that have fastpaths for getting the total number of bytes under a prefix. I don't really want a Store.getsize_and_count_prefix, but maybe some kind of Store.statistics_prefix? Probably not worth worrying about today.

[rabernat]

Thanks so much for this Tom!

The double listing will definitely add some latency. But we can work on that in a future PR.

I could imagine an API like

nitems, total_size = store.statistics(prefix, ("count", "size"))