When getting warnings from the retry_interceptors that RPCs failed, they
provide very little context as to what exactly failed.
I used this patch to try debugging
https://groups.google.com/g/etcd-dev/c/3hxxr9ktdpM.
To fix a bug in the retry logic caused when the auth token is cleared after receiving `ErrInvalidAuthToken` from the server and the subsequent call to `getToken` also fails due to some reason (eg. context deadline exceeded).
This leaves the client without a token and the retry will continue to fail with `ErrUserEmpty` unless the token is refreshed.
- ErrGRPCNotCapable("etcdserver: not capable") -> codes.FailedPrecondition (it will not autofix, it requires new version of server)
- ErrGPRCNotSupportedForLearner("etcdserver: rpc not supported for learner") -> codes.FailedPrecondition (as long as its learner, the call will not work)
- ErrGRPCClusterVersionUnavailable("etcdserver: cluster version not found during downgrade") -> codes.FailedPrecondition (backend does not contain the version (old etcd?) so retry will not help)
https://github.com/etcd-io/etcd/runs/2599598633?check_suite_focus=true
```
{"level":"warn","ts":"2021-05-17T09:55:30.246Z","logger":"etcd-client","caller":"v3/retry_interceptor.go:62","msg":"retrying of unary invoker failed","target":"etcd-endpoints://0xc000539880/#initially=[unix://localhost:m30]","attempt":0,"error":"rpc error: code = Unavailable desc = etcdserver: rpc not supported for learner"}
{"level":"warn","ts":"2021-05-17T09:55:30.270Z","logger":"etcd-client","caller":"v3/retry_interceptor.go:62","msg":"retrying
of unary invoker
failed","target":"etcd-endpoints://0xc000539880/#initially=[unix://localhost:m30]","attempt":1,"error":"rpc
error: code = Unavailable desc = etcdserver: rpc not supported for
learner"}`
```
clientv3 logs (especially tests) were poluted with unattributed to testing.T log lines:
```
{"level":"warn","ts":"2021-04-29T12:42:11.055+0200","logger":"etcd-client","caller":"v3/retry_interceptor.go:64","msg":"retrying of unary invoker failed","target":"etcd-endpoints://0xc0000fafc0/#initially=[unix://localhost:m10]","attempt":0,"error":"rpc error: code = ResourceExhausted desc = etcdserver: mvcc: database space exceeded"}
```
The reasons were 2 fold:
- Interceptors were copying logger before "WithLogger" could modify it.
- We were not propagating the loggers in a few testing contexts.
Old etcdserver which have not apply pr of #12165 will check auth token
even if the request is an Authenticate request.
If the client has a invalid auth token, it will not able to update it's
token, since the Authenticate has a invalid auth token.
This fix clear the auth token when encounter an ErrInvalidAuthToken to
talk with old version etcd servers.
Fix#12385 with #12165 and #12264
