Add user facing logging API for OpenTelemetry C++

[ThomsonTan]

Before opening a feature request against this repo, consider whether the feature should/could be implemented in the other OpenTelemetry client libraries. If so, please open an issue on opentelemetry-specification first.

Is your feature request related to a problem?
The OpenTelemetry Spec currently defines bridge-api which is not intended to be called by application developers. OpenTelemetry C++ needs create its own user facing C++ for application developer to instrument their code.

Describe the solution you'd like
Design a new set of logging API to support common scenario based on the bridge API.

Describe alternatives you've considered
Existing API has drawbacks to be used in production for large volume instrumentation.

Additional context
Add any other context about the feature request here.