`Client.dial` can be called multiple times. For example, from regular
`NewClient` and [from the `Maintenance`
wrapper](6d451ab61d/client/v3/maintenance.go (L81-L84)).
This ends up creating two (if `Client.dial` is called twice) independent
`credentials.Bundle` instances:
- the first one is wired into `PerRPCCredentials` callback in gRPC
client.
- the second one is in `Client.authTokenBundle` and receives the latest
auth token updates.
When using username/password authentication and the server is configured
to issue JWTs, token rotation logic breaks because of the above
`credentials.Bundle` confusion.
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.
