This commit adds logging to `exporter.log` to track how long it takes
to perform certain parts of `Gitlab::ImportExport::Saver`.

For Google Cloud Storage uploads, Labkit is not able to track the HTTP
timings because the underlying `httpclient` gem manages its own SSL
socket. This commit will make it possible to see which steps are
taking a long time:

1. Creating the .tar.gz file (`compress_duration_s`).
2. Assigning the file to the CarrierWave uploader
   (`assign_duration_s`).
3. Saving the file to object storage/local storage
   (`upload_duration_s`).
4. Number of bytes to upload (`upload_bytes`).

Relates to #349425

Changelog: added