Add missing include folders for example OTLP exporter

[ThomsonTan]

Added missing included folders and also renamed library target foo_library to otlp_foo_library in CMakeLists.txt. The name foo_library conflicts with the library in simple example.

gRPC dependency is still missing so I didn't add this exporter to the parent CMakeLists.txt, will fix it later.

[codecov]

Codecov Report

Merging #301 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #301   +/-   ##
=======================================
  Coverage   94.59%   94.59%           
=======================================
  Files         146      146           
  Lines        6629     6629           
=======================================
  Hits         6271     6271           
  Misses        358      358           

[lalitb]

LGTM.

[ThomsonTan]

Thanks. @reyang could you please also help approve and merge this?

[reyang]

Thanks. @reyang could you please also help approve and merge this?

CI is failing, need to get it fixed.

https://github.com/open-telemetry/opentelemetry-cpp/blob/master/CONTRIBUTING.md#how-to-receive-comments

By default I will ignore PRs that failed CI.

[reyang]

LGTM.