add push hook

[ClemChou000]

add push hook in order to execute some extra actions like reset registry or update metrics synchronously

[hagen1778]

Hey @KoyomiKun!

Have you tried using InitPushExt function and add hook to the writeMetrics func(w io.Writer) function?

[valyala]

Closing this pull request, since the extra actions can be performed inside writeMetrics callback, which is passed to InitPushExt and InitPushExtWithOptions, as @hagen1778 pointed out above. For example:

customMetricsSet := metrics.NewSet()
// add the needed metrics to customMetricsSet ...
// ...

writeMetrics := func(w io.Writer) {
  pushHook()  // call pushHook before sending the customMetricsSet metrics to pushURL
  customMetricsSet.WritePrometheus(w)
}
if err := metrics.InitPushExt(pushURL, pushInterval, extraLabels, writeMetrics); err != nil {
  panic(err)
}

If you still feel that the pushHook is needed as a separate callback, then try adding it to the PushOptions in a new pull request.