Printing number of keys in DB Stats

Summary: It is useful to print out number of keys in DB Stats

Test Plan:
./db_bench --benchmarks fillrandom --num 1000000 -threads 16 -batch_size=16

and watch the outputs in LOG files

Reviewers: MarkCallaghan, ljin, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24513