feat: add CreatedAt function

[fcgxz2003]

CreatedAt is the function to get creation time of probes.

Related Issue

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #2244 (1f5b832) into main (d13bc0e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2244   +/-   ##
=======================================
  Coverage   49.64%   49.64%           
=======================================
  Files         146      146           
  Lines       19505    19507    +2     
=======================================
+ Hits         9683     9684    +1     
- Misses       9226     9227    +1     
  Partials      596      596           

Flag	Coverage Δ
Object-compatibility-e2etests	?
e2etests	?
unittests	49.64% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
scheduler/networktopology/probes.go	88.00% <100.00%> (+0.32%)	⬆️

... and 1 file with indirect coverage changes

[gaius-qi]

LGTM