[fix] delete endpoint /debug

[Format-C-eft]

This MR proposes the removal of the default /debug endpoint, which is automatically registered by the net/http standard library. The /debug endpoint exposes debugging information that may include sensitive data, posing a potential security risk in production environments.

Key changes:

1. Removed the registration of the /debug handler via http.DefaultServeMux.

Benefits:

1. Eliminates a potential security vulnerability related to the exposure of debugging information.
2. Reduces the risk of data leakage in production environments.
3. Please review the changes and provide feedback.

[it-percona-cla]

All committers have signed the CLA.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.50%. Comparing base (dc46ed5) to head (ff3ab8d).
Report is 107 commits behind head on main.

Files with missing lines	Patch %	Lines
exporter/server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1099      +/-   ##
==========================================
- Coverage   70.88%   65.50%   -5.39%     
==========================================
  Files          28       29       +1     
  Lines        3569     3073     -496     
==========================================
- Hits         2530     2013     -517     
- Misses        904      922      +18     
- Partials      135      138       +3     

Flag	Coverage Δ
agent	65.50% <0.00%> (-5.39%)	⬇️

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