create --stats: populate bytes read/sent from store stats

[mr-raj12]

Description

On borgstore-backed repositories, borg create --stats always printed "Bytes read/sent from remote: 0".

rx_bytes and tx_bytes only exist on the legacy RemoteRepository. A modern repository doesn't have them, so getattr(repository, "rx_bytes", 0) returned 0 every time.

Rather than pulling two numbers out of repository.store.stats, this now reports the full store stats dict: call counts and timings per operation, load/store volume and throughput, and cache hit/miss numbers. Each entry gets its own "Store : " line in --stats output, and the full dict is included under store_stats in --json.

The stats are sampled after the cache is closed, since Cache.close() writes the chunks index to the repo and that traffic should be counted too. The create epilog documents what the store stats mean.

fixes #9405

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[ThomasWaldmann]

I think the last pack should already get flushed before the archive pointer is created.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.12%. Comparing base (230f6ef) to head (54dede3).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9866      +/-   ##
==========================================
+ Coverage   85.09%   85.12%   +0.03%     
==========================================
  Files          93       93              
  Lines       15450    15478      +28     
  Branches     2335     2343       +8     
==========================================
+ Hits        13147    13176      +29     
+ Misses       1604     1600       -4     
- Partials      699      702       +3     

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[mr-raj12]

I think the last pack should already get flushed before the archive pointer is created.

agreed!, the last pack is already flushed in Archives.create() before the pointer is written, so my comment was wrong on that. The real reason to sample after cache close is that Cache.close() writes the chunks index to the repo, and if we want that traffic counted too.
fixing comment and PR text.