I know disk space is cheap these days, but at 16MB/metric/<level-of-granularity>, it seems like your metric dataset would grow pretty quick. With just 10 metrics tracked daily, thats another gigabyte per week. Of course it does come with the benefit of maintaining all the raw data since you never roll up or aggregate the data...so the pros probably outweigh that con. :)
Redis stores everything in RAM, and RAM is not as cheap as disk. Adding GB's of RAM every week will quickly get rather expensive. But I guess you could dump old data to disk and load it back to Redis only when you need it. It might even compress well, depending on what the metrics track.
Isn't Redis still single-threaded for queries, but saving in the background? That seems a little risky: you've got your 100 million users setting bits in your bitsets and suddenly everything blocks for 10 seconds while old data is being loaded from disk.
It only really needs be captured like that using redis, when the collection period is up you could store it on disk, or even just the aggregate information on disk.
