https://github.com/googleapis/google-cloud-cpp/blob/da56bf943fd25c6a23b2c4272da8aacb96f81e82/google/cloud/opentelemetry/monitoring_exporter.h

We should move the factory function and the three options out of the otel_internal namespace.

I considered not moving out the options relevant to Google-defined metrics (CreateServiceTimeSeries). These options are only relevant to Google projects. I think we should release them too because:

• A Google project that wants to write to Google-defined metrics can exist outside of google-cloud-cpp
• The documentation is clear about who the audience for these options are