Discovered while investigating #68311: telemetry counter file extension can cause windows processes to panic. For example: this crash is from a new end-to-end test added in https://go.dev/cl/597278, without any changes to the x/telemetry logic.

https://ci.chromium.org/ui/p/golang/builders/try/x_telemetry-go1.21-windows-amd64/b8742848988042691361/test-results?q=ExactID%3Agolang.org%2Fx%2Ftelemetry%2Finternal%2Fcounter.TestConcurrentExtension+VHash%3Afb236f0bcc2ba24d&clean=&sortby=&groupby=

I don't think this is a theoretical or pathological setup. With the high concurrency of toolchain processes, it seems likely that Windows users will encounter crashes due to this bug. Furthermore, these crashes will only become more likely as we add more instrumentation. Therefore, marking as a release blocker.

CC @golang/telemetry