embed: only log stream error with debug level

[tangcong]

Disable noisy gRPC logs.

22:07:55 etcd1 | {"level":"warn","ts":"2020-10-20T22:07:55.500+0800","caller":"grpclog/grpclog.go:66","msg":"transport: http2Server.HandleStreams failed to read frame: read tcp 127.0.0.1:2379->127.0.0.1:62786: read: connection reset by peer"}
22:07:55 etcd1 | {"level":"info","ts":"2020-10-20T22:07:55.500+0800","caller":"grpclog/grpclog.go:51","msg":"transport: loopyWriter.run returning. connection error: desc = \"transport is closing\""}
22:07:56 etcd1 | {"level":"warn","ts":"2020-10-20T22:07:56.135+0800","caller":"grpclog/grpclog.go:66","msg":"transport: http2Server.HandleStreams failed to read frame: read tcp 127.0.0.1:2379->127.0.0.1:62789: read: connection reset by peer"}
22:07:56 etcd1 | {"level":"info","ts":"2020-10-20T22:07:56.136+0800","caller":"grpclog/grpclog.go:51","msg":"transport: loopyWriter.run returning. connection error: desc = \"transport is closing\""}

[niharkawli]

@tangcong @gyuho Is this fix also going to be merged to 3.4 branch? We are seeing these logs with latest 3.4.x - v3.4.18 as well.

[kkkkun]

@tangcong @gyuho Is this fix also going to be merged to 3.4 branch? We are seeing these logs with latest 3.4.x - v3.4.18 as well.

It do not cherrypick this PR to 3.4 branch.
https://github.com/etcd-io/etcd/blob/release-3.4/embed/config_logging.go#L198

Let me do this!