x/telemetry: file extension is not concurrency safe

[findleyr]

As in the description: extending the telemetry file is not concurrency safe.

Reproducible by making ./internal/counter.TestLarge concurrent:

unexpected fault address 0x7f206e535920
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x1 addr=0x7f206e535920 pc=0x5387b2]

goroutine 11378 gp=0xc0039d28c0 m=28 mp=0xc001508008 [running]:
runtime.throw({0x58307e?, 0x0?})
        /usr/local/google/home/rfindley/src/go/src/runtime/panic.go:1067 +0x48 fp=0xc00055cd98 sp=0xc00055cd68 pc=0x46c408
runtime.sigpanic()
        /usr/local/google/home/rfindley/src/go/src/runtime/signal_unix.go:914 +0x26c fp=0xc00055cdf8 sp=0xc00055cd98 pc=0x46dc4c
sync/atomic.(*Uint64).Load(...)
        /usr/local/google/home/rfindley/src/go/src/sync/atomic/type.go:169
golang.org/x/telemetry/internal/counter.(*Counter).add(...)
        /usr/local/google/home/rfindley/src/telemetry/internal/counter/counter.go:252
golang.org/x/telemetry/internal/counter.(*Counter).releaseLock(0xc00171d7c0, 0x7fffffff)
        /usr/local/google/home/rfindley/src/telemetry/internal/counter/counter.go:233 +0x312 fp=0xc00055cea8 sp=0xc00055cdf8 pc=0x5387b2
golang.org/x/telemetry/internal/counter.(*Counter).Add(0xc00171d7c0, 0x264)
        /usr/local/google/home/rfindley/src/telemetry/internal/counter/counter.go:178 +0x38a fp=0xc00055cf50 sp=0xc00055cea8 pc=0x5380ea
golang.org/x/telemetry/internal/counter.TestLarge.func2()
        /usr/local/google/home/rfindley/src/telemetry/internal/counter/counter_test.go:165 +0x118 fp=0xc00055cfe0 sp=0xc00055cf50 pc=0x53f3b8
runtime.goexit({})
        /usr/local/google/home/rfindley/src/go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc00055cfe8 sp=0xc00055cfe0 pc=0x473dc1
created by golang.org/x/telemetry/internal/counter.TestLarge in goroutine 17
        /usr/local/google/home/rfindley/src/telemetry/internal/counter/counter_test.go:162 +0x225

I will fix this soon, for [email protected].

CC @golang/telemetry

[findleyr]

Whoops, this appears to be an error in my test. I haven't yet demonstrated a bug.