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

…tion

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.

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

Fixes DataDog#2199

contrib/gofiber/fiber.v2/fiber.go

@@ -73,7 +73,7 @@ func Middleware(opts ...Option) func(c *fiber.Ctx) error {
 			tracer.Tag(ext.Component, componentName),
 			tracer.Tag(ext.SpanKind, ext.SpanKindServer),
 		)
-		span, ctx := tracer.StartSpanFromContext(c.Context(), cfg.spanName, opts...)
+		span, ctx := tracer.StartSpanFromContext(c.UserContext(), cfg.spanName, opts...)
 
 		defer span.Finish()