go-feature-flag feat: add dynamic flagset reloading without restart

Description

This PR implements dynamic flagset reloading for the relay-proxy, allowing flagsets to be added or removed without restarting the service. The implementation follows the requirements from issue #4288:

Add/remove flagsets without restart: Flagsets can now be dynamically added or removed by modifying the configuration file
Reject modifications to existing flagsets: If an existing flagset configuration is modified (beyond API key changes), the entire configuration change is rejected
Config file only: All changes are controlled through the configuration file, no API endpoints are added

Implementation Details

Config File Watcher: Added a file watcher using fsnotify that monitors the configuration file for changes
ReloadFlagsets Method: New method on FlagsetManager that:
- Validates existing flagsets haven't been modified
- Adds new flagsets
- Removes deleted flagsets
- Rejects the entire change if any existing flagset is modified
Thread-Safe: All flagset operations are protected with mutex locks
Comprehensive Tests: Added 9 test cases covering all scenarios

How to Test

Start the relay-proxy with flagsets configured
Modify the configuration file to add a new flagset
The new flagset should be available without restart
Try modifying an existing flagset configuration - it should be rejected
Remove a flagset from the config - it should be removed without restart

Closes issue(s)

Resolves #4288

Checklist

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

Dec 10 '25 21:12 thomaspoignant

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

Name	Link
Latest commit	91c91f827f26ec916e4e616bfd7f078724cd1480
Latest deploy log	https://app.netlify.com/projects/go-feature-flag-doc-preview/deploys/6939f2b75ef52b0008e1e6d1

Dec 10 '25 21:12 netlify[bot]

Codecov Report

:x: Patch coverage is 60.11236% with 71 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 84.61%. Comparing base (57caa82) to head (91c91f8). :warning: Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/relayproxy/config/config.go	0.00%	29 Missing :warning:
cmd/relayproxy/main.go	0.00%	24 Missing :warning:
cmd/relayproxy/service/flagset_manager.go	85.60%	14 Missing and 4 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4462      +/-   ##
==========================================
- Coverage   85.33%   84.61%   -0.73%     
==========================================
  Files         145      145              
  Lines        6056     6233     +177     
==========================================
+ Hits         5168     5274     +106     
- Misses        663      730      +67     
- Partials      225      229       +4

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