rename prometheus related flags'web.listen-address' to 'web.listen-port'

[paddie]

Currently the prometheus flags are under the web. prefix:

$ ./beat-exporter -help
Usage of ./beat-exporter:
  ...
  -web.listen-address string
        Address to listen on for web interface and telemetry. (default ":9479")
  -web.telemetry-path string
        Path under which to expose metrics. (default "/metrics")

I propose we put this under prom, and maybe sharpen the language a bit:

$ ./beat-exporter -help
Usage of ./beat-exporter:
  ...
  -prom.port string
        Port to listen on for web interface and telemetry. (default "9479") <-- leave out the quote
  -prom.path string
        Path under which to expose metrics. (default "/metrics")

Thoughts?

[shivas]

It follows prometheus and few most used exporters naming patterns, example: https://github.com/prometheus/node_exporter/blob/master/node_exporter.go#L71

[paddie]

I bow my heads to the prometheus precedent :)