Resolves open-telemetry#376

Use cases where this is necessary or useful:

1. Specify more than one resource in the telemetry: open-telemetry#579
2. Data coming from external source, e.g. AWS Metadata: open-telemetry#596 (comment)
   or open-telemetry#376 (comment)
3. Capturing HTTP headers: open-telemetry#376 (comment)
4. Structured stack traces: open-telemetry#2841
5. Payloads as attributes: open-telemetry/oteps#219 (comment)

This is a draft PR to see what the change looks like.

If this PR is merged it will be nice to follow it up with:
- A standard way of flattening maps and nested objects when converting from OTLP
  to formats that don't support maps/nested objects.
- Recommendations for semantic conventions to use/not use complex objects.

…ed anymore (we prohibit complex types for metrics)