Custom tracing provider does not work

[Nomango]

What happened:

迁移 tracing 后端服务的时候想保证原来老的 tracing 暂时可用，但是 tracing middleware 默认使用了 global provider，导致 provider 的配置项不生效。

比如：

http.Middleware(
	tracing.Server(tracing.WithTracerProvider(provider1)),
	tracing.Server(tracing.WithTracerProvider(provider2)),
)

只有 provider2 会生效

What you expected to happen:

provider1 和 provider2 同时生效

How to reproduce it (as minimally and precisely as possible):

同上

Anything else we need to know?:

Environment:

• Kratos version (use kratos -v): v2.5.3
• Go version (use go version): v1.19
• OS (e.g: cat /etc/os-release): macOS 13.3.1
• Others: