When looking at the effectiveness of the WiredTiger cache, it can be useful to know how often we have cache misses (i.e., how often a request for data from the cache requires us to read a page from the file.

The cache miss rate can be computed from existing FTDC statistics as:

miss rate = wt cache pages read into cache / wt cache pages requested from cache

We should add this to t2 as a derived statistic.

Note: we should report the "miss rate" rather than a "hit rate".