Fix multiport support

[thomaspoignant]

Description

When merging #1502 we had some errors in the the way the echo instances where managed.
This PR fixes the issue.

Changes include

• Bugfix (non-breaking change that solves an issue)

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have followed the contributing guide

[netlify]

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

Name	Link
🔨 Latest commit	85510f2
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/65bd151add6e2e0008743d80

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (c66b203) 87.67% compared to head (85510f2) 87.75%.

Files	Patch %	Lines
cmd/relayproxy/api/server.go	86.36%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1503      +/-   ##
==========================================
+ Coverage   87.67%   87.75%   +0.08%     
==========================================
  Files          77       77              
  Lines        3626     3627       +1     
==========================================
+ Hits         3179     3183       +4     
+ Misses        364      361       -3     
  Partials       83       83              

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[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud