fix: Wrong metrics timestamp

[amirkaws]

Please provide the issue number

Issue number: 207

Summary

This PR addresses the incorrect metrics timestamp

Changes

Please provide a summary of what's being changed

Timestamp for the context has change to generate a new time every time context flushes and serialises EMF metrics to CloudWatch logs

User experience

Please share what the user experience looks like before and after this change

Checklist

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• Meets tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

Acknowledgment

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[codecov-commenter]

Codecov Report

Base: 56.35% // Head: 56.35% // No change to project coverage 👍

Coverage data is based on head (636de73) compared to base (f854e95).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #209   +/-   ##
========================================
  Coverage    56.35%   56.35%           
========================================
  Files           41       41           
  Lines         1778     1778           
========================================
  Hits          1002     1002           
  Misses         776      776           

Flag	Coverage Δ
unittests	56.35% <100.00%> (ø)

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

Impacted Files	Coverage Δ
...rc/AWS.Lambda.Powertools.Metrics/Model/Metadata.cs	97.91% <100.00%> (-0.05%)	⬇️
...ls.Metrics/Serializer/JsonNamingPolicyDecorator.cs	57.14% <0.00%> (ø)
...da.Powertools.Logging/Internal/PowertoolsLogger.cs	91.01% <0.00%> (+0.05%)	⬆️

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