MINOR: Add startup-duration logging to help with Kafka startup performance tuning

[jiafu1115]

Sometimes we need to know how long Kafka takes to start in order to make certain decisions—for example, tuning configuration parameters or estimating how much time operations will require. However, it’s quite inconvenient to manually look for the timestamp of the first log line and then the first log line hint the server has started, and calculate the difference.
So this PR adds a direct log output for the startup duration.

Log the total startup time after successful server startup: "Kafka startup completed in ${startupDurationMs} ms"

Benefits:

1. Quickly identify slow startups
2. Easily know startup duration directly in logs without repeat manual calculation

[github-actions]

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