Allow building as a header-only library with CMake

[lidavidm]

I'm working on integrating OpenTelemetry in a CMake-based project using ExternalProject. I want my project to be able to use OpenTelemetry as a header-only library, using just api/, or optionally directly linking to OpenTelemetry (libopentelemetry_trace etc). In the former case, I'd like to not build any of the static libraries to save build time, however, there's not a good way to do this.

Is your feature request related to a problem?
No

Describe the solution you'd like
Being able to disable the build of the ext/sdk/exporters subprojects would be ideal, e.g. -DWITH_SDK=OFF -DWITH_EXT=OFF -DWITH_EXPORTERS=OFF.

Describe alternatives you've considered
CMake kind of lets you do something like this:

  externalproject_add_step(opentelemetry_ep install-headers
                           COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> --target api/install
                           LOG true)
  externalproject_add_steptargets(opentelemetry_ep install-headers configure)
  externalproject_add_stepdependencies(opentelemetry_ep install-headers
                                       opentelemetry_ep-configure)

However, that --target is specific to the Ninja CMake generator - if you're using Makefiles, it instead has to be --build <BINARY_DIR>/api --target install not --build <BINARY_DIR> --target api/install. Trying to embed all this logic is brittle and I'd rather have the option to configure the build itself. Is this reasonable?

Additional context
N/A

[lalitb]

That would be a common scenario, and it would make sense to support building only the api project.

[lalitb]

@lidavidm Solution suggested by you looks good - basically adding cmake options to enable/disable building projects selectively. Or just to add WITH_API_ONLY to enable building only API project ( as this would be required for building instrumentation libraries). Wanted to check if you have plans to contribute on this, else we can let someone pick this up :)

[lidavidm]

@lalitb Sorry for the delay - I would like to but I'm still working with my org to make sure I can sign the CLA. I'm hoping that'll be in the next couple days, else someone else can of course take a look.

[lalitb]

Thank you @lidavidm for your willingness to contribute. There is no immediate hurry, you can pick it up once CLA is signed :)