[google-auth-library-java] cleanup: create internal request type identifier for ComputeEngineCredentials getMetadataResponse()

context: https://github.com/googleapis/google-auth-library-java/pull/1586#discussion_r1960466711

both metric and logging can benefit from this piece of information passing down. A minor refactor should achieve this.

从 ComputeEngineCredentials.getMetadataResponse() 开始，阅读已链接 pull request 的讨论，以获取所需的上下文。跟踪元数据请求如何到达指标和日志记录代码；完成的标准是传递一个内部请求类型标识符，以便两者都能使用它。

由索引模型根据 Issue 内容生成。