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

[prateek]

Ported from https://github.com/m3db/m3x/pull/186

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[coveralls]

Coverage decreased (-1.7%) to 92.613% when pulling e54f4213222323eae45d511e82c56489e9cb4dae on prateek:prateek/tally-prometheus-listen-address into d0a004a77fc9f0b9733e3afe638994d18da13f3d on uber-go:master.

[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.