Fix a test failure that happens with TSAN enabled.
Fix a test failure that happens with TSAN enabled.

In the corresponding test, "++" is not thread safe, and we have 3 threads that do this concurrently. Instead, we have each thread own a single parameter for increment, then no race condition will be introduced.

PiperOrigin-RevId: 321251011

google/eDistantObject,google/eDistantObject,google/eDistantObject