[AsperforMias]

Description

Fixes #3804

dubbo-go does not write timestamp into the instance metadata, so Dubbo Admin cannot read a register time and shows dubbo-go instances as UnRegistered.

This PR writes timestamp in createInstance, in milliseconds. Admin parses it with time.UnixMilli and Dubbo Java writes milliseconds, so a value in seconds would show up in Admin as a 1970 date. Only this field uses milliseconds; other timestamps in dubbo-go stay in seconds.

Verified end to end with Nacos application-level discovery and Dubbo Admin (develop):

• develop: no timestamp, registerState is UnRegistered and registerTime is empty.
• this PR: timestamp=1794496237887, registerState is Registered and registerTime is the current time.

The test checks that the written timestamp is in milliseconds and falls between the times taken before and after registration.

Checklist

• I confirm the target branch is develop
• I have run make fmt to format my code
• I have run make test to run local tests
• I have added tests that prove my fix is effective or that my feature works

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.72%. Comparing base (60d1c2a) to head (6edae75).
⚠️ Report is 1041 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #3807       +/-   ##
============================================
+ Coverage    46.76%   58.72%   +11.95%     
============================================
  Files          295      492      +197     
  Lines        17172    40433    +23261     
============================================
+ Hits          8031    23744    +15713     
- Misses        8287    14910     +6623     
- Partials       854     1779      +925     

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[Alanxtl]

lgtm