Fix cmake find config of fluentd and update corresponding README

[zurex]

Description

This PR raised for address issue #477, navigate to the issue to get more details.

Changes

• Update the exporters/fluentd/cmake/opentelemetry-cpp-fluentd-config.cmake.in to make sure it works as expected.
• Add new section Incorporating into an existing CMake Project in the README of fluentd for better guide.

Test && Validation

• Test in local linux system and can't provide a unitest (at least i dont know the correct way).

Existing Behavior

Use find_package(opentelemetry-fluentd CONFIG REQUIRED) get empty OPENTELEMETRY_CPP_FLUENTD_LIBRARY_DIRS and OPENTELEMETRY_CPP_FLUENTD_INCLUDE_DIRS

New Behavior

Now OPENTELEMETRY_CPP_FLUENTD_LIBRARY_DIRS and OPENTELEMETRY_CPP_FLUENTD_INCLUDE_DIRS works as expected:

• OPENTELEMETRY_CPP_FLUENTD_LIBRARY_DIRS: [opentelemetry-fluentd::trace, opentelemetry-fluentd::logs]
• OPENTELEMETRY_CPP_FLUENTD_INCLUDE_DIRS: same as the value of OPENTELEMETRY_CPP_INCLUDE_DIRS

[linux-foundation-easycla]

• ✅login: zurex / (7d88319)

The committers listed above are authorized under a signed CLA.

[ThomsonTan]

I am not sure opentelelemetry-fluentd is a good name here, because it is a fluentd exporter for opentelemetry-cpp.

[lalitb]

I believe target name is coming from project-name defined in CMakeLists.txt, and not defined in this PR. This PR just fixes that name to be properly used in OPENTELEMETRY_CPP_FLUENTD_LIBRARY_DIRS.

But agree, opentelemetry-cpp-fluentd is better naming, we can have separate PR to fix that.

[zurex]

@ThomsonTan as lalitb mentioned, this PR only do a bug fix and make sure the find_package could works as expected. If you want, i could create a separate PR to make sure the project name follow the same name pattern.