fix(tracing): Restore OTel tracing support

[hairyhenderson]

Description

In #1503, the code that used to call s.otelService.Init was accidentally removed. This restores that code.

Here's proof of spans once again being sent to a local Alloy/Tempo setup:

Note that I have not written a unit test to cover this, as it's quite difficult to do so with how the OTel SDK is configured currently.

I have some ideas on how tracing support can be made more flexible (through use of autoexport primarily, and some other changes). I'll issue a PR for that some other time.

Closes issue(s)

Fixes #2479

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	349be1b
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/6705b61f7b93f0000859e757

[sonarcloud]

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 SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.18%. Comparing base (f45f594) to head (349be1b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/relayproxy/api/server.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2482      +/-   ##
==========================================
+ Coverage   85.66%   86.18%   +0.52%     
==========================================
  Files         100      100              
  Lines        3523     3526       +3     
==========================================
+ Hits         3018     3039      +21     
+ Misses        382      360      -22     
- Partials      123      127       +4     

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[thomaspoignant]

Thanks a lot for this PR, I have no idea why I removed it.

I will release a new version containing the fix.