otelfiber: Add custom attributes support

[kaptinlin]

We can add some custom attributes to span, if we need to customize

app := fiber.New()
app.Use(
	otelfiber.Middleware(
		otelfiber.WithTracerProvider(provider),
		otelfiber.WithCustomAttributes(func(ctx *fiber.Ctx) []attribute.KeyValue {
			return []attribute.KeyValue{
				attribute.Key("http.query_params").String(ctx.Request().URI().QueryArgs().String()),
			}
		}),
	),
)

[gaby]

@kaptinlin Can you take a look at the merge conflict

[gaby]

Lgtm

[kaptinlin]

@kaptinlin Can you take a look at the merge conflict

updated