feat(observability): implement url.domain attribute

[diegomarquezp]

This Pull Request implements the url.domain observability attribute in the gax-java library. This attribute records the 'nominal' domain where the request is intended to go (e.g., service.googleapis.com), distinct from server.address which reflects the actual destination (which could be localhost or a proxy).

The attribute is now recorded in both attempt-level spans and metrics for both gRPC and HTTP/JSON transports, provided that the serviceName is available in the ClientContext.

[gemini-code-assist]

Code Review

This pull request enables the urlDomain attribute for observability across all transport types by updating ClientContext to derive it from the service name and universe domain. It also modifies ApiTracerContext to include this attribute in both attempt and metrics attributes and adds relevant unit and integration tests. A review comment identifies a potential issue where a null universe domain could result in a malformed string like "serviceName.null", suggesting a check for both values before concatenation.