standardize on instrumentation detection patterns

[toumorokoshi]

We should consider a standard for marking objects as instrumented by opentelemetry. There are a couple conventions that exist now:

• setting the _is_instrumented attribute
• setting the _is_instrumented_by_opentelemetry attribute

We should choose one, and maybe provide some patterns to reduce the code duplication.

[lzchen]

I like _is_instrumented_by_opentelemetry in case other libraries want to "instrument" as well it avoids collisions.

[github-actions]

This issue was marked stale due to lack of activity. It will be closed in 30 days.

[codeboten]

Closing in favour of open-telemetry/opentelemetry-python-contrib#439