Address spec compliance issue (SetName method is deprecated and removed)

[maxgolov]

SetName method is now removed in the mainline base class, based on Log spec update here:
open-telemetry/opentelemetry-cpp#1383
open-telemetry/opentelemetry-cpp@280f546

This causes the contrib fluentd build break when the code is compiled with latest OpenTelemetry SDK!

List of changes:

• Code change : remove override attribute in fluentd log/recordable.h . But keep the method for now. Need to figure out what to do with it in the long run. One possibility is to shim the method to SetAttribute("name", value) in a way that won't be affecting the code / customers that might still be using it.

• Code change : resolve compilation warnings in examples if HAVE_CONSOLE_LOG has been defined as a build option in the upper level makefile.

• NEW - non-invasive build infra option : allow to build fluentd exporter inside the main build tree via add_directory. Main idea here is borrowed from nlohmann-json project CMakeLists.txt. If fluentd exporter is plugged into the build via add_directory into main OpenTelemetry C++ SDK, then don't really need to post a separate package of it. And don't need to provide own config. As a result - artifacts get deployed as part of the main project install, in one build loop rather than in two build loops. Much faster. The other option (clone and build as main project) still there, without any functional changes to its behavior. I'll supply a separate README.md for that "fast deployment" later. That's what we use in our Docker image builds / CI pipeline elsewhere.

[lalitb]

Thanks for the PR. While we are doing this, should we also update the otel-cpp version used internally (if not already installed) here -

opentelemetry-cpp-contrib/exporters/fluentd/cmake/opentelemetry-cpp.cmake

Line 2 in 42d8252

set(opentelemetry-cpp-tag "75c2a8f7a6bf81d9799e76804473609cc236b7be") #to incorporate PR#1325

Else, the build may fail.

[maxgolov]

Else, the build may fail.

Done. Updated to latest.