contrib/gofiber/fiber.v2: use UserContext in Middleware for span creation

[tonyduburque]

What does this PR do?

The Middleware was incorrectly using c.Context() instead of c.UserContext() to start spans. This could break the propagation of the context defined by other middlewares earlier in the chain.

Motivation

This change ensures that tracer.StartSpanFromContext uses the correct context provided by Fiber, preserving context propagation and ensuring proper tracing behavior.

Fixes #2199

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

[rarguelloF]

could you please write a test reproducing the issue? 🙏

[tonyduburque]

There is an exemple in the linked issue.
But here is a simpler example :

package main

import (
	"github.com/gofiber/fiber/v2"
	"golang.org/x/net/context"
	fibertrace "gopkg.in/DataDog/dd-trace-go.v1/contrib/gofiber/fiber.v2"
)

func initUserContext() fiber.Handler {
	return func(c *fiber.Ctx) error {
		ctx := context.WithValue(c.UserContext(), "key", "value")
		c.SetUserContext(ctx)

		return c.Next()
	}
}

func checkUserContext() fiber.Handler {
	return func(c *fiber.Ctx) error {
		_, ok := c.UserContext().Value("key").(string)
		if !ok {
			return fiber.ErrInternalServerError
		}

		return c.Next()
	}
}

func defaultHandler(c *fiber.Ctx) error {
	return c.SendStatus(fiber.StatusOK)
}

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

	app.Use(initUserContext())       // Init UserContext with some key value
	app.Use(fibertrace.Middleware()) // Add Datadog middleware
	app.Use(checkUserContext())      // Check if UserContext is correctly set

	app.Get("/", defaultHandler)

	if err := app.Listen(":3000"); err != nil {
		panic(err)
	}
}

The expected outcome is a response with a status code of 200, but instead, a response with a status code of 500 is returned.

If the fibertrace middleware is removed, a response with a status code of 200 is obtained as expected.

[tonyduburque]

I modified the UserContext test to include the necessary checks