Add unit tests for RPC related code

[seeflood]

What would you like to be added:
Understand the implementation of the rpc module and add unit tests for it, so that the test coverage of the module reaches 80%
You can ask @MoonShining for help if you encounter any problems in the process.
The code path is in components/rpc
In addition, there are rpc related codes in pkg/grpc/api.go and pkg/runtime/runtime.go files

Reference materials:rpc design document

After completing this improvement, you will learn the principle of Layotto supporting rpc, how Layotto and mosn are integrated to reuse service mesh capabilities

chinese:
看明白rpc模块的实现原理并为rpc相关模块补充单测，使模块的单测覆盖率达到80%
过程中遇到问题可以问 @MoonShining 。
代码路径在 components/rpc
另外pkg/grpc/api.go 和pkg/runtime/runtime.go 文件里也有rpc相关的代码

参考资料：rpc设计文档

完成这项优化后，你将学习到Layotto支持rpc的原理、Layotto是如何和mosn整合到一起复用service mesh能力的

Why is this needed:
Currently the test coverage of rpc component is not high:

[seeflood]

@tianjipeng Hi,you can reply here
It seems a github rule that I can't assign it to you until you reply here

[tianjipeng]

Ok，please assign to me

[seeflood]

Ok, already assigned.