Is google-api-core dependency actually needed?

[micwoj92]

Describe your environment.
Opencensus 0.11.4

Steps to reproduce.
Check code for google-api-core imports.

What is the expected behavior?
Some imports

What is the actual behavior?
No imports

Additional context.
I've seen pull request #1075 which tries to remove this dependency, but @aabmass said:

Ah, so this won't work because of one actual use of protobuf library here:

opencensus-python/opencensus/tags/propagation/binary_serializer.py

Line 21 in 5ad2a7c

from google.protobuf.internal.encoder import _VarintBytes

However google.protobuf.internal.encoder is provided by protobuf package and not google-api-core. If there is something that I understand incorreclty, please let me know, thanks.