OpenTelemetry API instrumentation improvements

[trask]

Improvements to interoperability between OpenTelemetry API and auto-instrumentation.

• Subclass SpanContext and ensure users only get our subclass, which makes wrapping/unwrapping much more efficient. (pending https://github.com/open-telemetry/opentelemetry-java/issues/1314, also see initial attempt #513)
• Need to add interop with new CorrelationContext
  • Why does EntryValue need its own class instead of using String? (opened https://github.com/open-telemetry/opentelemetry-java/issues/1334)
• Look carefully over API for any other recommendations that could make interop smoother

[trask]

Closing. The first item didn't go where I thought it would (see valid concerns raised in #513). The Second item is done.