Expose the `addr` bit of prometheus' start_http_server() in the config

[tadzik]

~~listen_port being passed on to prometheus.start_http_server makes it a whole listen_address, rather than just a port, making configs look confusing when you actually need not-just-a-port in there.~~

~~Perhaps it'd be worth to rename it, keeping the existing one as a deprecated alias for a while?~~

metrics in the configuration only exposes port as a configurable, making it tricky to listen on non-standard addresses if needed.

[tulir]

prometheus_client.start_http_server's first argument seems to be specifically a port as an integer, not an address? The second argument is addr, making that configurable might be useful

As for renaming config fields, the config update system exists specifically to support legacy field names without any fallbacks in the actual code.

[tadzik]

Hmm, indeed – seems like we've been misusing it for quite a while, and it's been working due to a happy accident more than anything else...