Update OTLP to 0.20.0

[tigrannajaryan]

OTLP 0.20.0 was released: https://github.com/open-telemetry/opentelemetry-proto/releases/tag/v0.20.0

It is also the release candidate for upcoming 1.0.0.

Please update the OTLP exporter to use OTLP 0.20.0.

[tigrannajaryan]

@open-telemetry/python-maintainers can you please take a look and tell if v0.20.0 works for you? We are planning to declare OTLP 1.0 next week and any changes after that are going to be much harder (if not impossible).