Could we export `opentelemetry_otlp_recordable` headers?

[owent]

Is your feature request related to a problem?

I want to implement a special OTLP exporter to extend the standard OTLP exporters,but I need to control the recordable objects of OTLP exporters. But I found I cann't reuse recordables with cmake because the installation exclude the headers of OTLP recordables.

install(
  DIRECTORY include/opentelemetry/exporters/otlp
  DESTINATION include/opentelemetry/exporters
  FILES_MATCHING
  PATTERN "*.h"
  PATTERN "otlp_recordable.h" EXCLUDE)

I just wonder why we exclude otlp_recordable.h here, and can we also install it?

Describe the solution you'd like

Remove PATTERN "otlp_recordable.h" EXCLUDE above.

[ThomsonTan]

the header file was excluded in below PR, probably we should discuss to get it back for installation. @esigo

#923

[lalitb]

Yes I think this is valid use-case, we can bring this header file as part of installation. I can fix this next week, unless someone wants to raise a PR to fix this earlier?