[prometheus-reporter] Add ability to infer listen port based on env var

[prateek]

Ported from m3db/m3x#186

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[coveralls]

Coverage decreased (-1.7%) to 92.613% when pulling e54f421 on prateek:prateek/tally-prometheus-listen-address into d0a004a on uber-go:master.

[xichen2020]

Would it be more clear to name this PortType with static and dynamic being the two values? Static ports should always be loaded from config, and dynamic ports should always be resolved from env vars.

[xichen2020]

Hm this is kinda confusing, e.g., you can have a dynamicListenAddress that defines its port in the config file.

[xichen2020]

Technically you don't really need resolved since addr == "" && !resolved is always false.

[xichen2020]

nit: make the err errMissingPort.

[xichen2020]

Hm if this is always nonzero, why not make it a non-pointer?

[xichen2020]

nit: net.JoinHostPort might be more portable.

[jeromefroe]

I thought about this some more, instead of adding another configuration option, how would you feel if we modify the prometheus configuration in the service itself? For example, we could change:

server.Run(server.RunOptions{
		ConfigFile: *configFile,
})

to

var cfg config.Configuration
if err := xconfig.LoadFile(&cfg, *configFile, xconfig.Options{}); err != nil {
			fmt.Fprintf(os.Stderr, "unable to load %s: %v", runOpts.ConfigFile, err)
			os.Exit(1)
}

// update config.Configuration.Metrics.PrometheusReporter based on env variable

server.Run(server.RunOptions{
		Config: config,
})

I think this approach would address both of our concerns: we wouldn't need to munge the config files at runtime and we wouldn't need to include an additional configuration option in tally.