CI benchmarks return NUM_KEYS to previous size (#10649)

Summary:
Larger size is necessary to stress levels 2, 3 of LSM tree

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10649

Reviewed By: ajkr

Differential Revision: D39744515

Pulled By: jay-zhuang

fbshipit-source-id: 62ff097bfbfdfc26ff1e6290e1e3b71506b7042c

.circleci/config.yml

@@ -120,7 +120,7 @@ commands:
     steps:
       - run:
           name: "Test low-variance benchmarks"
-          command: ./tools/benchmark_ci.py --db_dir /tmp/rocksdb-benchmark-datadir --output_dir /tmp/benchmark-results --num_keys 50000
+          command: ./tools/benchmark_ci.py --db_dir /tmp/rocksdb-benchmark-datadir --output_dir /tmp/benchmark-results --num_keys 10000000
           environment:
             LD_LIBRARY_PATH: /usr/local/lib
             # How long to run parts of the test(s)