client.go: Degrade log level when passing unavailable endpoint to client (#1852)

[sre-bot]

cherry-pick #1852 to release-3.0

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What problem does this PR solve?

when some endpoint passing into the client is unavailable
in calling NewClient() will log the error log, but the service is
supported to be still available if one of the endpoints can work.
error level log will be too serious and confuse the user.

What is changed and how it works?

client.go: Degrade log level when passing unavailable endpoint to client

Check List

Tests

• Manual test (add detailed scripts or steps below)
  log after this pr:

[2019/10/28 18:51:04.975 +08:00] [INFO] [client.go:144] ["[pd] create pd client with endpoints"] [pd-address="[localhost:9999,localhost:2379]"]
[2019/10/28 18:51:04.987 +08:00] [WARN] [client.go:200] ["[pd] failed to get cluster id"] [url=http://localhost:9999] [error="rpc error: code = Unavailable desc = all SubConns are in TransientFailure, latest connection error: connection error: desc = \"transport: Error while dialing dial tcp [::1]:9999: connect: connection refused\""] [errorVerbose="rpc error: code = Unavailable desc = all SubConns are in TransientFailure, latest connection error: connection error: desc = \"transport: Error while dialing dial tcp [::1]:9999: connect: connection refused\"\ngithub.com/pingcap/pd/client.(*client).getMembers\n\t/Users/huangjiahao/go/pkg/mod/github.com/july2993/pd@v1.1.0-beta.0.20191028103916-ec504e3dce8f/client/client.go:235\ngithub.com/pingcap/pd/client.(*client).initClusterID\n\t/Users/huangjiahao/go/pkg/mod/github.com/july2993/pd@v1.1.0-beta.0.20191028103916-ec504e3dce8f/client/client.go:197\ngithub.com/pingcap/pd/client.(*client).initRetry\n\t/Users/huangjiahao/go/pkg/mod/github.com/july2993/pd@v1.1.0-beta.0.20191028103916-ec504e3dce8f/client/client.go:184\ngithub.com/pingcap/pd/client.NewClient\n\t/Users/huangjiahao/go/pkg/mod/github.com/july2993/pd@v1.1.0-beta.0.20191028103916-ec504e3dce8f/client/client.go:157\nmain.main\n\t/Users/huangjiahao/go/src/github.com/pingcap/tidb-binlog/cmd/pd.go:10\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:203\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:1357"]
[2019/10/28 18:51:04.994 +08:00] [INFO] [client.go:252] ["[pd] switch leader"] [new-leader=http://127.0.0.1:2379] [old-leader=]
[2019/10/28 18:51:04.994 +08:00] [INFO] [client.go:163] ["[pd] init cluster id"] [cluster-id=6752802479515318396]

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

[lhy1024]

/run-all-tests

[lhy1024]

/run-integration-common-test