[CMake] Discovery of grpc_cpp_plugin does not work with ALIAS targets

[igxactly]

It would be good if it works like alias target fallback of protoc. I know that the root cause is, from the fact that gRPC's cmake is not well written. But IMO it would be better to have it.

Describe your environment

• otel-cpp: latest (tried v1.22.2 and same code in main branch HEAD)
• compiler: GCC 15 (Fedora 42), 11 (RHEL 8)
• cmake: 3.29.x with ninja
• os: linux

Steps to reproduce

• Add absl, protobuf, gRPC as FetchContent
• Set gRPC:: namespaced ALIAS targets to gRPC's
  (as it is not defined so when it is used with add_subdirectory and similar things)
• Configure & build it

I am attaching full cmake usage example all the way below.

What is the expected behavior?

Even with non-imported but aliased targets, it would be better to work without problem.

What is the actual behavior?

The (path) variable gRPC_CPP_PLUGIN_EXECUTABLE which is from gRPC::grpc_cpp_plugin becomes empty so protoc step of OTel-gRPC protos fail.

Additional context

It would be better if this gRPC cpp plugin discovery steps

opentelemetry-cpp/cmake/opentelemetry-proto.cmake

Lines 181 to 186 in c406864

	if(TARGET gRPC::grpc_cpp_plugin)
	project_build_tools_get_imported_location(gRPC_CPP_PLUGIN_EXECUTABLE
	gRPC::grpc_cpp_plugin)
	else()
	find_program(gRPC_CPP_PLUGIN_EXECUTABLE grpc_cpp_plugin)
	endif()

works like this protoc discovery steps.

opentelemetry-cpp/CMakeLists.txt

Lines 401 to 412 in c406864

	if(TARGET protobuf::protoc)
	if(CMAKE_CROSSCOMPILING AND Protobuf_PROTOC_EXECUTABLE)
	set(PROTOBUF_PROTOC_EXECUTABLE ${Protobuf_PROTOC_EXECUTABLE})
	else()
	project_build_tools_get_imported_location(PROTOBUF_PROTOC_EXECUTABLE
	protobuf::protoc)
	# If protobuf::protoc is not a imported target, then we use the target
	# directly for fallback
	if(NOT PROTOBUF_PROTOC_EXECUTABLE)
	set(PROTOBUF_PROTOC_EXECUTABLE protobuf::protoc)
	endif()
	endif()

And below is the example of CMake-FetchContent usage.

    message(STATUS "gRPC: Using FetchContent")
    FetchContent_Declare(
        absl
        GIT_REPOSITORY https://github.com/grpc/grpc.git
        GIT_TAG        v1.39.1
        SOURCE_SUBDIR third_party/abseil-cpp
        EXCLUDE_FROM_ALL
        OVERRIDE_FIND_PACKAGE
    )
    FetchContent_Declare(
        protobuf
        GIT_REPOSITORY https://github.com/grpc/grpc.git
        GIT_TAG        v1.39.1
        SOURCE_SUBDIR third_party/protobuf/cmake
        EXCLUDE_FROM_ALL
        OVERRIDE_FIND_PACKAGE
    )
    FetchContent_Declare(
        gRPC
        GIT_REPOSITORY https://github.com/grpc/grpc.git
        GIT_TAG        v1.39.1
        EXCLUDE_FROM_ALL
        OVERRIDE_FIND_PACKAGE
    )

    set(gRPC_ABSL_PROVIDER "package")
    set(gRPC_PROTOBUF_PROVIDER "package")
    set(ABSL_PROPAGATE_CXX_STD ON)
    set(RE2_BUILD_TESTING OFF)
    set(gRPC_BUILD_TESTS OFF)

    FetchContent_MakeAvailable(absl)
    FetchContent_MakeAvailable(protobuf)
    FetchContent_MakeAvailable(gRPC)

    ## grpc does not offer namespaced targets, so we create them
    add_library(gRPC::grpc++ ALIAS grpc++)
    add_library(gRPC::grpc++_reflection ALIAS grpc++_reflection)
    add_executable(gRPC::grpc_cpp_plugin ALIAS grpc_cpp_plugin)

    FetchContent_Declare(
        opentelemetry-cpp
        GIT_REPOSITORY https://github.com/open-telemetry/opentelemetry-cpp.git
        GIT_TAG v1.22.2
        OVERRIDE_FIND_PACKAGE
    )

    set(WITH_OTLP_GRPC ON)
    set(WITH_OTLP_HTTP ON)
    set(WITH_OTLP_FILE ON)
    set(WITH_NO_DEPRECATED_CODE ON)

    set(BUILD_TESTING OFF)
    set(WITH_EXAMPLES OFF)
    set(WITH_BENCHMARK OFF)
    
    FetchContent_MakeAvailable(opentelemetry-cpp)

[dbarker]

@igxactly Thanks for logging the issue. #3533 was merged recently and adds support for fetching protobuf and grpc. With the main branch you can now just fetch opentelemetry-cpp and gRPC/protobuf will be fetched implicitly if you set WITH_OTLP_GRPC=ON.

[dbarker]

Closing as this is addressed by #3533