x/telemetry: download the upload config lazily

[findleyr]

This is a follow up to #68946. To fix that issue for go 1.23.1, we did the absolutely narrowest fix to suppress the telemetry config download if the telemetry mode is not "on".

However, we should also make the download lazy, so that we download the config once a week when there is actual work to upload, rather than once a day when the upload process runs.

We could also consider back-porting this fix to 1.23.2, as I think it will be a safe change.

[gabyhelp]

Related Issues and Documentation

• x/telemetry: allow to use custom config download logic #67726

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