Add Len and Capacity methods for ring buffer

[xigang]

What type of PR is this?
/kind feature

What this PR does / why we need it:

Add Len and Capacity methods for Informer metrics to calculate numberOfPendingNotifications and sizeOfRingGrowing.

PR: kubernetes/kubernetes#129160

KEP: https://github.com/kubernetes/enhancements/tree/master/keps/sig-api-machinery/4346-informer-metrics

type eventHandlerMetrics struct {

  // number of pending data
  numberOfPendingNotifications GaugeMetric

  // size of RingGrowring data
  sizeOfRingGrowing GaugeMetric

  // how long processing an item from informer reflector
  prcoessDuration  HistogramMetric
        
}

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Release note:

NONE

[k8s-ci-robot]

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[xigang]

/cc @deads2k @liggitt @aojea PTAL

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[xigang]

/assign @liggitt @aojea

[liggitt]

does anything ever shrink this buffer?

[xigang]

@liggitt No, this ring buffer implementation (RingGrowing) only grows and never shrinks. Looking at the code:

1. The buffer starts with an initialSize specified in NewRingGrowing.
2. When the buffer becomes full (r.readable == r.n), the WriteOne method doubles the size (newN := r.n * 2).
3. There are no methods that reduce the size of the buffer.

If shrinking capability is needed, a method can be added to shrink the buffer when it becomes too empty (for example, when readability falls below a certain threshold, such as 25% of the capacity).

Thank you, liggitt, for reviewing the code.：）

[xigang]

ping @deads2k Please take a look.

[xigang]

@liggitt Can this PR be approved?