[API] Clarify expectations for C++ exceptions, and usage of noexcept methods.

[marcalff]

Several methods in the API are flagged as noexcept.

This is desirable, because adding instrumentation to an application (i.e., calling opentelemetry-cpp apis) should not make the application less stable.

In particular, any failure in the opentelemetry-cpp sdk or exporters should not propagate the exception up, taking the application down.

To comply with the noexcept contract, methods in the SDK implementation should never raise exceptions.

According to clang-tidy reports, this is not always the case.

This part should be revisited, to clarify expectations, and enforce the SDK implementation complies.

cc @msiddhu