refactor(xatu): allow setting metrics addr via cli flag

[mattevans]

Allows setting --metrics-addr=:XXXX via cli flag for the following Xatu pkgs...

• cannon
• cl-mimicry
• discovery
• mimicry
• relay-monitor
• sentry
• server

Takes precedence over any metricsAddr set in config.

Example (without flag)

go run main.go sentry                                                                                                                                                            
...
INFO[0000] Serving metrics at :9090                      module=sentry

Example (with flag)

go run main.go sentry --metrics-addr=:8765                                                                                                                                                            
...
INFO[0000] Overriding metrics address                    address=":8765" module=sentry
INFO[0000] Serving metrics at :8765                      module=sentry