move _timestamp logic to the internal process

[kptkin]

Fixes WB-10592

Description

What does the PR do?

This PR cleans up the partial_history_callback and moves the _timestamp logic to the internal process

Testing

How was this PR tested?

If this fix is correct, the test is that all the existing tests don't break

[codecov]

Codecov Report

Merging #4087 (8fe532a) into master (917dd7e) will increase coverage by 7.48%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4087      +/-   ##
==========================================
+ Coverage   75.26%   82.74%   +7.48%     
==========================================
  Files         255      255              
  Lines       32267    32266       -1     
==========================================
+ Hits        24286    26699    +2413     
+ Misses       7981     5567    -2414     

Flag	Coverage Δ
functest	54.91% <100.00%> (+0.03%)	⬆️
unittest	73.44% <100.00%> (+7.47%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wandb/sdk/wandb_run.py	91.18% <ø> (+2.83%)	⬆️
wandb/sdk/interface/interface.py	96.69% <100.00%> (+0.01%)	⬆️
wandb/integration/tensorboard/monkeypatch.py	87.65% <0.00%> (-4.94%)	⬇️
wandb/sdk/lib/git.py	79.01% <0.00%> (ø)
wandb/sdk/wandb_settings.py	94.83% <0.00%> (+0.97%)	⬆️
wandb/sdk/service/server_sock.py	92.89% <0.00%> (+1.01%)	⬆️
wandb/sdk/lib/sock_client.py	91.36% <0.00%> (+1.43%)	⬆️
wandb/sdk/wandb_init.py	85.43% <0.00%> (+2.52%)	⬆️
wandb/sdk/internal/meta.py	90.79% <0.00%> (+3.06%)	⬆️
... and 28 more