Raise shard count to 10 for tensorflow/python/kernel_tests:metrics_test

The test was sometimes taking over six minutes to run in asan mode,
causing it to hit the 5 minute timeout.   Setting the shard count
to 6 was inufficient, but setting it to 10 brought the runtime
down to about 3:30 in the worst case over 100 runs.

PiperOrigin-RevId: 183449941