fix panic concurrent map writes when constructing metadata

[kbiits]

Description

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking changes (change that is not backward-compatible and/or changes current functionality)

Closes issue(s)

Resolve #1662

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

[netlify]

✅ Deploy Preview for go-feature-flag-doc-preview canceled.

Name	Link
🔨 Latest commit	979ad5d
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/65fabd28ae8c2a0008869e21

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.84%. Comparing base (5b0868c) to head (979ad5d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1663   +/-   ##
=======================================
  Coverage   87.84%   87.84%           
=======================================
  Files          77       77           
  Lines        3669     3669           
=======================================
  Hits         3223     3223           
  Misses        360      360           
  Partials       86       86           

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[thomaspoignant]

Hey, good catch 🙌
Thanks a lot for this contribution, I will be available in the next version (probably this week).