kvserver: clear rac2 token metrics prior to integration testing

[kvoli]

TestFlowControl.*V2 tests assert on exact counters. This can be problematic if benign deltas occur while setting up the test, such a send queue forming when adding a new learner, but being quickly resolved.

Clear the token metrics prior to commencing these tests, in order to prevent flakes that result from such deltas in setup.

Fixes: #132642 Release note: None