Update integration tests to use grpc

[Shaddoll]

What changed?

• Update integration tests to use grpc
• Remove nil check for responses with errors (somehow grpc returns empty struct instead of nil for error response)

Why?

• Tchannel is going to be deprecated
• We have features only available in grpc

How did you test it?
integration tests

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.79%. Comparing base (2655dc3) to head (7b57c4a).
Report is 1 commits behind head on master.

Additional details and impacted files

see 7 files with indirect coverage changes

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