metrics: add support of push metrics by multiple url

[dmitryk-dk]

It is a simple improvement. Added function that can handle different URLs for push metrics to sets up periodic push by defined URLs for metrics obtained by calling writeMetrics callback with the given interval.

I think there can be another improvement. When we can define struct like

type Push struct {
   URL string 
   Interval time.Duration
   ExtraLabels string
}

We can define different intervals and extra labels per each URL. It may be helpful to define different write callbacks, but it is just my thoughts.

[dmitryk-dk]

Closed as unneeded