usage

• Start by commandline
  Between numbers of domain,using comma to split. Start method are as follows:
  go build sslcert-exporter && ./sslcert-exporter -domains=www.xxx.com,www.xx.com

• Start by container
  Change Dockerfile "CMD" field, replace target domains to which you care

Implement

• The custom interface named "Collector" contains 2 methods: describe and collect
• This implement don't need usinig roundrobin to get metrics data periodically. When we interview the metircs api(e.g http://localhost:9999/metrics),the collect method is called. And in collect method, the "checkHttps" method is called automatically.