PushConfig represents a struct package uses for pushing
metrics to remote destination. Having a structure
helps to extend functionality in the future, without
touching the signature of existing functions.
For example, `PushConfig` supports custom HTTP headers
via `Headers` param.
Updates https://github.com/VictoriaMetrics/metrics/issues/52
Updates https://github.com/VictoriaMetrics/metrics/issues/36
Co-authored-by: hagen1778 <roman@victoriametrics.com>
