backend, net: log client versions

[djshow832]

What problem does this PR solve?

Issue Number: close #369

Problem Summary:
Some client versions are in the connection attributes. If we can log them once the connection is created, it might be helpful for troubleshooting.

What is changed and how it works:

• Move the log connected to backend after the connection is established.
• Add client names and versions in the log.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

[2023/11/08 16:25:28.157 +08:00] [INFO] [main.proxy.conn] [client/client_conn.go:52] [connected to backend] [connID=5] [client_addr=127.0.0.1:65006] [addr=0.0.0.0:6000] [client_version=8.1.0] [client_name=libmysql] [capability=CLIENT_LONG_PASSWORD|CLIENT_LONG_FLAG|CLIENT_CONNECT_WITH_DB|CLIENT_LOCAL_FILES|CLIENT_PROTOCOL_41|CLIENT_SSL|CLIENT_TRANSACTIONS|CLIENT_SECURE_CONNECTION|CLIENT_MULTI_STATEMENTS|CLIENT_MULTI_RESULTS|CLIENT_PLUGIN_AUTH|CLIENT_CONNECT_ATTS|CLIENT_PLUGIN_AUTH_LENENC_CLIENT_DATA|CLIENT_DEPRECATE_EOF] [proxy-protocol=false] [backend_addr=127.0.0.1:4000]

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-08 09:52:30.548804007 +0000 UTC m=+3637948.135914136: ☑️ agreed by xhebox.