x/telemetry: simplify state across counters, reports, mode

[findleyr]

While working on implementing gotelemetry clean (#63142), I started to look into the consequences of deleting state from the telemetry config dir. Here are a few observations:

• Each counter file, when it is created, looks for the presence of a report file (.json), to detect whether the user is 'new'. If no report file is found, the counter expiry is increased by a week.
• When creating reports to upload, they are collected by expiry, and a local report is created for any non-empty expiry before today. An upload report is created at the same time, if the telemetry mode is "on".
• When the upload pass is run, it collects any upload reports that haven't been uploaded, and uploads them.

I see several problems with this:

• If a user runs gotelemetry clean it will essentially reset their state to that of a new user, and new counters will skip an upload week (their expiry will be set far into the future).
• If a user runs gotelemetry off I don't see anything preventing old upload reports from being uploaded, though I may have missed something. That's a problem: we should never upload anything if the Mode is off.
• If a user runs gotelemetry on after a local report has been created (e.g. a few weeks after local collection started), we can potentially find local counters that expire the next day, not at least 7 days into the future as we suggest.
• If a user runs gotelemetry off and then subsequently runs gotelemetry on, we'll potentially upload new counter files sooner than expected, due to the presence of historical reports.

If I'm not misunderstanding, I think these are all bugs that need to be fixed, and I don't think we can fix them with the current data model, due to the complicated state interactions:

• counter expiry reflects the state of the local dir at the time the counter was created.
• upload reports reflect the state of the mode at the time the report was created.

I think the simplest and best fix is to associate an explicit date with the mode file, recording when the mode was set to "on". Then we can easily guarantee that we won't upload data before that date, and won't submit the first upload before T+7.

Originally posted by @findleyr in #63142 (comment)

[gopherbot]

Change https://go.dev/cl/530995 mentions this issue: all: store an asof date in mode, and use it to guard uploads

[gopherbot]

Change https://go.dev/cl/532175 mentions this issue: all: remove 7 day offset for new uploads

[findleyr]

This is done.