transport/client: Return status code Unknown on missing grpc-status

[mbissa]

#8548 changed the grpc status code returned on missing grpc-status header to Internal. This PR changes it back to Unknown as that is the expected behavior.

See https://github.com/grpc/grpc/blob/master/doc/statuscodes.md for more details. Here Unknown is defined as follows:

Unknown error. For example, this error may be returned when a Status value received 
from another address space belongs to an error space that is not known in this 
address space. Also errors raised by APIs that do not return enough error information
may be converted to this error.

RELEASE NOTES:

• transport/client : Return status code Unknown on missing grpc-status.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.23%. Comparing base (e816736) to head (7a7b596).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8702      +/-   ##
==========================================
- Coverage   83.29%   83.23%   -0.06%     
==========================================
  Files         416      416              
  Lines       32267    32267              
==========================================
- Hits        26876    26858      -18     
- Misses       4017     4028      +11     
- Partials     1374     1381       +7     

Files with missing lines	Coverage Δ
internal/transport/http2_client.go	92.61% <100.00%> (+0.19%)	⬆️

... and 20 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

We want to first undo the status code change made in #8548, do the release and then make the other status code change after the release.

[dfawley]

Please update the PR title and make sure the commit matches [EDIT: when merging].

Also please add more explanation for this change in the PR description. In this case we should explain why we are making this change and link to the PR that introduced the behavior change we are reverting.

[easwars]

LGTM, modulo actions required for Doug's final comments.

[dfawley]

@mbissa

Please update the PR title and make sure the commit matches [EDIT: when merging].

This doesn't seem to have happened. So now the commit says "unparsable grpc-status" but we are still returning Internal on an unparsable status.

Also please add more explanation for this change in the PR description. In this case we should explain why we are making this change and link to the PR that introduced the behavior change we are reverting.

This can still be done. Please edit the description and PR title appropriately.

[easwars]

I updated the title and the description for posterity.