fix(count): fix Acquire/Reset data race in Tracker (PIPE-1231)

[Dylan-M]

Proposed Change

Acquire() and Reset() can run at the same time, and both touch the tracker's done/resume channels. Reset() reassigns them under t.mu, but Acquire() closed the done channel without taking t.mu, so the two race.

Acquire() now takes t.mu for that close. It does this only on the final permit, so normal acquires stay lock-free.

How to validate

• Automated: go test -race ./generator/count/ passes, including TestReset_ConcurrentWithAcquire (the failing signal before this change). Acquire and Reset are at 100% coverage.
• Manual: run a generator with a finite count (e.g. count: 1000) and confirm it emits exactly that many records and stops.

Checklist

• Changes are tested
• CI has passed

[Dylan-M]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• ci: run the test suite under the race detector (PIPE-1223) #272
• fix(output): drain buffered records on TCP/UDP Stop (PIPE-1230) #271
• fix(count): fix Acquire/Reset data race in Tracker (PIPE-1231) #270 👈 (View in Graphite)
• fix(generators): own backoff timing in the worker to fix ExponentialBackOff data race (PIPE-1222) #269
• main

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