[Feat] Better control of threads executed by opentelemetry-cpp

[marcalff]

Context

The opentelemetry-cpp uses threads internally, for various components.

For example:

• the batch processor uses a thread to process records from the span buffer.
• the periodic metric reader uses a thread to trigger periodic collection, and a child thread to collect observable metrics.
• the log processor uses a thread to process records from a log buffer.
• every exporter based on CURL uses a thread internally, to process HTTP requests and responses.

Problem

An application that links with the opentelemetry-cpp library may have additional constraints, and may want to control at a very low level how the code executes within a thread.

For example:

• the application may want to name threads, to be visible in the operating system under an application controlled name.
• the application may need to initialize thread local storage, in particular when custom code is executed within a thread spawned by opentelemetry. Typically, callbacks for observable metrics may need TLS keys in place to execute properly.
• the application may need to control thread priorities, or bind internal opentelemetry threads to some CPUs.
• the application may need to control the current network namespace, when a thread opens a network connection.

Currently, there is no way to implement any of this from an application that uses opentelemetry-cpp.

[lalitb]

This was discussed earlier, with some proposal here - https://github.com/open-telemetry/opentelemetry-cpp/issues/1822#issuecomment-1333400422