Golang x/telemetry: download the upload config lazily

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.

Comment From: gabyhelp

Related Issues and Documentation

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

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}