[BUILD] OTLP GRPC exporter fails to build with non default GRPC install

[marcalff]

Using the OTLP GRPC exporter.

GRPC is installed in a non default location, for example /opt/grpc-1.46.4

In CMake,

gRPC_DIR is set to /opt/grpc-1.46.4/lib/cmake/grpc

During the build:

malff@localhost.localdomain:build-otelcpp-grpc> make
Consolidate compiler generated dependencies of target opentelemetry_proto
[  0%] Building CXX object CMakeFiles/opentelemetry_proto.dir/generated/third_party/opentelemetry-proto/opentelemetry/proto/collector/trace/v1/trace_service.grpc.pb.cc.o
In file included from /home/malff/CODE/MARC_GITHUB/build-otelcpp-grpc/generated/third_party/opentelemetry-proto/opentelemetry/proto/collector/trace/v1/trace_service.grpc.pb.cc:6:
/home/malff/CODE/MARC_GITHUB/build-otelcpp-grpc/generated/third_party/opentelemetry-proto/opentelemetry/proto/collector/trace/v1/trace_service.grpc.pb.h:25:10: fatal error: 'grpcpp/impl/codegen/async_generic_service.h' file not found
#include <grpcpp/impl/codegen/async_generic_service.h>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

C++ code is properly generated from protobuf files.

The generated C++ code fails to build, because GRPC headers can not be found, as they are in a non default location, not added in the makefiles.

Suggested fix:

diff --git a/cmake/opentelemetry-proto.cmake b/cmake/opentelemetry-proto.cmake
index 1aa1ba03..7addca8b 100644
--- a/cmake/opentelemetry-proto.cmake
+++ b/cmake/opentelemetry-proto.cmake
@@ -285,6 +285,10 @@ else() # cmake 3.8 or lower
   target_link_libraries(opentelemetry_proto INTERFACE ${Protobuf_LIBRARIES})
 endif()
 
+if(WITH_OTLP_GRPC)
+  target_link_libraries(opentelemetry_proto PUBLIC gRPC::grpc++)
+endif()
+
 if(BUILD_SHARED_LIBS)
   set_property(TARGET opentelemetry_proto PROPERTY POSITION_INDEPENDENT_CODE ON)
 endif()

[owent]

The generated grpc codes in opentelemetry_proto are splited into opentelemetry_proto_grpc in #2097 .We can not link gRPC::grpc++ into opentelemetry_proto directly on ELF platform, but we can include the INTERFACE_INCLUDE_DIRECTORIES of gRPC::grpc++ in opentelemetry_proto_grpc.