Record cache hit/miss and upload ratios

[DanTup]

It'd be useful to see percentages of when caches are hit/missed and also uploaded. A cache that is good today might get broken in future (eg. every build writes into that folder) and go unnoticed. It'd be great if this was called out somewhere ("Hey, this cache is missing 99% of the time!").

[fkorotkov]

@edigaryev I think we have it already. Just need to expose it in the API and show somehow.