[ISSUE] some nessary information is not In ServiceMetadata

[vio-lin]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.7.3
• Operating System version: win7
• Java version: jdk8

Steps to reproduce this issue

1. define a service Interface like this.

public interface HelloBOMService {
  PersonResponseType get(PersonRequestType request) throws Exception;
}

2. getServiceDefinition and print Method Parameter

 ServiceDefinition serviceDefinition = ServiceDefinitionBuilder.build(HelloBOMService.class);
        System.out.println(serviceDefinition.getMethods().get(0).getParameters());

3. the methodDefition return an empty list

4. Extends TypeBuilder to construct Protobuf typeBuilder and add SPI file.

@SPI
public interface TypeBuilder {

2. Use serviceDefition.getTypes() to obtains all TypeDefinitions. It is nessary to know which TypeDefinitionBuilder build this TypeDefition.When TypeBuilder generate same ServiceDefinition
   use different logic.

Expected Result

We could get parameter type list from MethodDefition.
We should know which Builder build this TypeDefition.