Add metrics

@soundcloud/iss @ohm  @juliusv 
By default, a summary (histogram) will forget observations older than 10 minutes:

https://github.com/prometheus/client_golang/blob/master/prometheus/summary.go#L56

This makes sense if you want to measure high-frequency events such as user-facing HTTP traffic, where you don't want super-old observations to affect your current histogram. In the case of doozerd backup case though, you are doing backups so infrequently that you will never have more than 1 observation in your summary before the summary gets flushed. And after it's flushed, you won't export any timeseries for the summary anymore, because the summary now has 0 observations to summarize.

You can choose to either set the age threshold really high (see https://github.com/prometheus/client_golang/blob/master/prometheus/summary.go#L110) so that multiple observations happen in that time. Or, as I would recommend, you could just simplify this by using a gauge instead of a summary and just track the latest backup time. I'd call the metric `doozerd_backup_last_backup_duration_seconds` then.