x/telemetry/counter/countertest: improve ergonomics of countertest.Read

[findleyr]

While investigating a frequent gopls flake (#68659), I encountered several API problems with the countertest package that make it hard (or really impossible) to use correctly.

I believe the sequence of events that leads to the flake in question are:

1. TestMain calls countertest.Open, because it must be called once during program execution.
2. The test setup first performs an initial increment of counter values, since we must inc before the first Read as ReadCounter(counter.New("unknownCounter")) returns an error rather than succeeding and returning 0.
3. ReadCounter calls rotate1 internally. I'm not sure why. Since TestMain may have occurred ~minutes ago, it's quite likely that the day has actually changed in the meantime, and so the file rotation invalidates the previous counters.

IMO, all three of these are indicative of an API problem:

1. We should probably be able to call countertest.Open inside each test (albeit not in parallel), to increase test isolation.
2. ReadCounter(counter.New("unknownCounter")) should return the zero value.
3. ReadCounter should probably not call rotate1, but even if it does, countertest should not be subject to day shifts. We should either stop time, or let the countertest user control the progress of time. Otherwise, no test can assert on the output of Read.

At the very least, we should fix (3), but it would be good to fix (1) and (2) as well.

[gopherbot]

Change https://go.dev/cl/647436 mentions this issue: gopls/internal/test/integration: reduce flakes in TestTelemetryPrompt

[rsc]

Was just looking into this, but it appears to be fixed by CL 599075, which is exactly what I was going to do for (3).
I don't believe (1) and (2) should be necessary.

[rsc]

I am a little confused now because I notice that CL 599075 was submitted in July 2024. Perhaps it's just not thorough enough.

[findleyr]

@rsc that's not quite related. Open still calls rotate1, which maps the file initially.

counter.Read also calls rotate1. I don't actually know why. It's the rotate1 in counter.Read that causes the switch to a different file.