x/telemetry/upload: convert old counter files to local report files

[hyangah]

Currently, if a counter file is more than 21 days old, the uploader deletes the file without converting it to local report.
That can leave a mysterious gap in the local telemetry viewer. It will be hard to tell whether there were no counters collected or the uploader ran too late. Let's keep converting the file to the local json report before deletion.

I think removing https://go.googlesource.com/telemetry/+/refs/heads/master/internal/upload/reports.go#49 is sufficient.
https://go.googlesource.com/telemetry/+/refs/heads/master/internal/upload/reports.go#119 is already preventing upload of too old counters.

[gopherbot]

Change https://go.dev/cl/528296 mentions this issue: internal/upload: generate local report for obsolete count files