[Build] Build break when using WITH_OTLP_HTTP=ON, WITH_OTLP_GRPC=OFF

[marcalff]

When compiling with the OTLP HTTP exporter but not the OTLP GRPC exporter,
the build fails:

In file included from .../opentelemetry-cpp/opentelemetry-cpp-1.8.0/exporters/otlp/src/otlp_grpc_utils.cc:4:
.../opentelemetry-cpp/opentelemetry-cpp-1.8.0/exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_utils.h:6:10: fatal error: grpcpp/grpcpp.h: No such file or directory
    6 | #include <grpcpp/grpcpp.h>
      |          ^~~~~~~~~~~~~~~~~
compilation terminated.

Suggested fix for exporter/otlp/CMakeLists.txt:

• do not add otlp_grpc_utils.cc in the opentelemetry_otlp_recordable library
• add otlp_grpc_utils.cc in the opentelemetry_exporter_otlp_grpc library instead ? (to investigate)

[marcalff]

@ThomsonTan

If I understand correctly, file src/otlp_grpc_utils.cc should not be in library opentelemetry_otlp_recordable.

Now the question remains to decide in which library to add file src/otlp_grpc_utils.cc instead:

• opentelemetry_exporter_otlp_grpc_client ?
• opentelemetry_exporter_otlp_grpc ? (trace specific, so probably not)
• opentelemetry_exporter_otlp_grpc_log ? (logs specific, so probably not)
• opentelemetry_exporter_otlp_grpc_metrics ? (metrics specific, so probably not)
• other new library for common GRPC code ?

Please advise on how to fix this.

[sirzooro]

What about compiling this file only when WITH_OTLP_GRPC=ON? It could stay in opentelemetry_otlp_recordable lib.

Edit: you can also wrap contents of this file in something like #ifdef WITH_OTLP_GRPC, so during compilation with WITH_OTLP_GRPC=OFF file would be empty for compiler.

[esigo]

bazel uses otlp_grpc_utils library:
https://github.com/esigo/opentelemetry-cpp/blob/82d8cb49cdc2104ef5db1f73cf362a6c7a8a76a2/exporters/otlp/BUILD#L49-L64

[ThomsonTan]

For bazel build, the new util code goes to a separate new lib, probably we could do the same for CMake? opentelemetry_exporter_otlp_grpc_client looks another acceptable place.

https://github.com/open-telemetry/opentelemetry-cpp/blob/main/exporters/otlp/BUILD#L50

[lalitb]

Have raised a PR by adding it to opentelemetry_exporter_otlp_grpc_client, it can be separate library too. I don't have strong preference for either.