Add a benchmark for the shard_cache_mutex option.

Change-Id: Ia96ef5928efd57ca0804de0f553a4718377a4d2c
Reviewed-on: https://code-review.googlesource.com/c/36452
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit e7c832772ec55f495927bfa4526d2fc17f0381be)
Reviewed-on: https://code-review.googlesource.com/c/36453
1 file changed