[chalmerlowe]

Summary

Comparison prototype (Clone 1 of #18869) implementing Approach 1: threading default_host explicitly into google.api_core._observability and gapic_v1.method.wrap_method / method_async.wrap_method instead of mutating client_options.api_endpoint.

Changes on top of #18869

1. Explicit default_host parameter (google-api-core & gapic-generator):
   • Adds default_host: Optional[str] = None to _extract_endpoint_attributes, get_otel_interceptor, get_otel_async_interceptor, apply_otel_async_channel_interceptors, and gapic_v1.method.wrap_method / method_async.wrap_method.
   • Leaves self._client_options on Transport completely untouched (from_dict conversion only; no copying or mutation of api_endpoint).
2. Pre-constructed channel guard (_ignore_credentials):
   • Sets self._default_host = None if self._ignore_credentials else self._host so pre-constructed grpc.Channel / grpc.aio.Channel instances do not falsely report DEFAULT_HOST when api_endpoint is unset.
   • Updates construct_observability_client in test_span_compliance.py to pass a callable channel factory (_insecure_channel_factory) so local Showcase gRPC compliance tests exercise the primary host-managed path (self._ignore_credentials = False).
3. Upfront dict conversion:
   • Converts dict client_options upfront via client_options_lib.from_dict(client_options).

[gemini-code-assist]

Code Review

This pull request introduces support for passing a resolved 'default_host' fallback parameter to transport wrappers and OpenTelemetry interceptors in 'google-api-core' and generated GAPIC clients, ensuring correct endpoint attribute extraction when 'api_endpoint' is unset. Feedback on the changes highlights a critical issue in the generated compatibility check ('_compat.py.j2'), where checking 'kwdefaults' for 'default_host' will always evaluate to False because it is a positional-or-keyword argument; using 'inspect.signature' is recommended instead.

[gemini-code-assist]

The current check for default_host support relies on __kwdefaults__. However, default_host is defined as a standard positional-or-keyword argument (not a keyword-only argument) in gapic_v1.method.wrap_method. Therefore, __kwdefaults__ will be None, and SUPPORTS_DEFAULT_HOST will always evaluate to False, silently disabling the feature in generated clients.\n\nUsing inspect.signature is a robust and standard way to check if the parameter is supported.

import inspect\n\nSUPPORTS_DEFAULT_HOST = \"default_host\" in inspect.signature(gapic_v1.method.wrap_method).parameters