[WIP] Fix bytesSent tag not working in logging middleware

[Claude]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

---

This section details on the original issue you should resolve

<issue_title>🐛 [Bug]: bytesSent tag not working in logging middleware.</issue_title>
<issue_description>### Bug Description

bytesSent tag is always 0

How to Reproduce

Run demo code

output:

/test====0====

Expected Behavior

bytesSent tag has an actual value

Fiber Version

v3.1.0

Code Snippet (optional)

package main

import (
  "github.com/gofiber/fiber/v3"
  "github.com/gofiber/fiber/v3/middleware/logger"
)

func main() {
  app := fiber.New()

  app.Use(logger.New(logger.Config{
      Format: "${url}====${bytesSent}====\n",
  }))

  app.Get("/test", func(c fiber.Ctx) error {
      return c.SendString("test")
  })

  log.Fatal(app.Listen(":3000"))
}

Checklist:

• I agree to follow Fiber's Code of Conduct.
• I have checked for existing issues that describe my problem prior to opening this one.
• I understand that improperly formatted bug reports may be closed without explanation.</issue_description>

Comments on the Issue (you are @claude[agent] in this section)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.86%. Comparing base (9fd194f) to head (f0bd9d9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4227      +/-   ##
==========================================
- Coverage   91.24%   90.86%   -0.38%     
==========================================
  Files         123      123              
  Lines       11910    15227    +3317     
==========================================
+ Hits        10867    13836    +2969     
- Misses        655     1001     +346     
- Partials      388      390       +2     

Flag	Coverage Δ
unittests	90.86% <100.00%> (-0.38%)	⬇️

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[liaohongxing]

@claude[agent] Please estimate the performance of len(c.Response().Body())