Remove dependency on GUARDIAN_FLAG_DOGSTATSD_ADDRESS

[cleanshavenalex]

When Guardian is deployed with a URL for GUARDIAN_FLAG_DOGSTATSD_ADDRESS
And that URL cannot resolve
Then Guardian should use a NullReporter
And Log that it is not collecting metrics
https://github.com/dollarshaveclub/guardian/blob/master/cmd/guardian/main.go#L69-L73

If the URL GUARDIAN_FLAG_DOGSTATSD_ADDRESS cannot be resolved for any reason Guardian will enter a CrashLoop, this may be expected and acceptable. However I do believe that if URL GUARDIAN_FLAG_DOGSTATSD_ADDRESS experiences issues after Guardian is successfully started, Guardian will fail to handle requests resulting in 'backend not found' ingress response to clients.