Ensure all experimental properties are marked "experimental"

[jack-berg]

We want to make sure that we only stabilize with properties which are stable parts of the specification. Properties which represent experimental portions of the spec should be marked experimental

Blocked by the definition of experimental, which #142 attempts to define how experimental properties are demarcated.

A few properties come to mind, but we should do a thorough analysis

• Instrumentation config, i.e. .instrumentation.*
• OTLP file exporter, i.e. .tracer_provider.processors[].batch.exporter.otlp_file.*
• Prometheus exporter, i.e. .meter_provider.readers[].pull.exporter.prometheus.*