fix(core): Upload tensorboard event files in core

[timoffex]

Description

Implements watching and uploading tfevents files for TensorBoard.

Testing

• Unit tests in tb_test.go
• Manual repro as in WB-17261

[codecov]

Codecov Report

Attention: Patch coverage is 56.66667% with 52 lines in your changes are missing coverage. Please review.

Please upload report for BASE (spr/timoffex/main.fixcore-upload-tensorboard-event-files-in-core@11f8b93). Learn more about missing BASE report.

Additional details and impacted files

@@                                         Coverage Diff                                         @@
##             spr/timoffex/main.fixcore-upload-tensorboard-event-files-in-core    #7677   +/-   ##
===================================================================================================
  Coverage                                                                    ?   73.08%           
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  Files                                                                       ?      502           
  Lines                                                                       ?    54309           
  Branches                                                                    ?        0           
===================================================================================================
  Hits                                                                        ?    39693           
  Misses                                                                      ?    14192           
  Partials                                                                    ?      424           

Flag	Coverage Δ
func	41.39% <ø> (?)
system	63.49% <12.50%> (?)
unit	52.15% <56.66%> (?)

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

Files	Coverage Δ
core/pkg/server/handler.go	69.71% <0.00%> (ø)
core/pkg/server/stream.go	95.89% <82.35%> (ø)
core/pkg/server/tb.go	57.62% <53.46%> (ø)