Directory cli fixes

[nothingmuch]

Various fixes for the directory's CLI interface, spun out of #1026

1. actually use metrics_listen_addr by invoking serve_metrics_tcp from main
2. make metrics_listen_addr optional in the config struct, and only start the metrics listener if it's not Some
3. make all CLI args optional, use them as overrides only when explicitly provided

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[coveralls]

Pull Request Test Coverage Report for Build 18357327396

Details

• 0 of 17 (0.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.09%) to 84.13%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-directory/src/main.rs	0	4	0.0%
payjoin-directory/src/config.rs	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
payjoin-directory/src/config.rs	2	0.0%

Totals
Change from base Build 18288677845:	-0.09%
Covered Lines:	8954
Relevant Lines:	10643

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💛 - Coveralls

[spacebear21]

ACK 44e74b0 but I'm also open to just merging #1026 directly as-is since these changes have already been reviewed over there.