humanize.Time() drops precision resulting in some events reporting they took
"now" time to complete. Using time.Duration.String() results in accurate
duration being reported.
Fixes#13905
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.
qsyqian