Zipkin exporter v2 api support for protobuf format

[robwknox]

Description

Adds support for the Zipkin Exporter v2 API's protobuf format.

Includes 2 auto-generated files related to the protobuf format:

• exporter/opentelemetry-exporter-zipkin/src/opentelemetry/exporter/zipkin/zipkin_pb2.py the core buffer code
• exporter/opentelemetry-exporter-zipkin/src/opentelemetry/exporter/zipkin/zipkin_pb2.pyi derived mypy stub file to aid development

File gen command (based on google docs):

./protoc -I=exporter/opentelemetry-exporter-zipkin/src/opentelemetry/exporter/zipkin/ \
  --python_out=exporter/opentelemetry-exporter-zipkin/src/opentelemetry/exporter/zipkin/ \
  --mypy_out=exporter/opentelemetry-exporter-zipkin/src/opentelemetry/exporter/zipkin/ \
  zipkin.proto

Fixes #1175

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Breaking change is minor - the dict opentelemetry.exporter.zipkin.SPAN_KIND_MAP structure has changed but I suspect this is only being internally used by the module.

How Has This Been Tested?

Manual testing with local zipkin server doing comparisons between JSON & protobuf data exports to ensure they identically matched. Plus full tox run against envs:

• 3.5.10
• 3.6.12
• 3.7.9
• 3.8.6
• 3.9.0
• pypy3.6-7.3.1

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[ocelotl]

LGTM 👍