Protobuf 4.21.0 does not work with current generated opentelemetry-proto

[aabmass]

Someone on slack reported that protobuf 4.21.0 broke their OTLP exporter, with the cause being protocolbuffers/protobuf#10051. In the meantime, it looks like that protobuf version was yanked https://pypi.org/project/protobuf/4.21.0/ (I'm assuming for this reason).

It looks like opentelemetry-proto package has proto > 3.13.0 which allows 4.x releases.. we really should have it pinned to the major version 3 i.e. proto ~= 3.13. Either way, we will need to re-generate the protobuf messages for the new major version (see protocolbuffers/protobuf#10051 (comment))

opentelemetry-python/opentelemetry-proto/setup.cfg

Line 45 in 709afdd

protobuf>=3.13.0

[aabmass]

Since the version was release, I think the best action for the upcoming release is to pin to major version 3. Once protobuf unyanks or cuts a new release, we will need to regenerate and up the dependency to pin major version 4.

[aabmass]

Dup of #2717