How to replace pattern with shell command's output using matched pattern

Use case at hand: replace Unix timestamps with date/time in a log file:

[1630720618] unbound[63495:0] info: 127.0.0.1 foo.bar.com AAAA IN
[1630720618] unbound[63495:1] info: 127.0.0.1 foo.bar.com A IN
...

The shell command I'd apply is `date -r <timestamp>`. How would you
perform the substitution?

The desired output is:

[Sat Sep 4 03:56:58 CEST 2021] unbound[63495:0] info: 127.0.0.1 foo.bar.com AAAA IN
[Sat Sep 4 03:56:58 CEST 2021] unbound[63495:1] info: 127.0.0.1 foo.bar.com A IN
...

Thanks,
Life.

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