src/telemetry/metrics/availability.js
function AvailabilityMetric () {
  if (!(this instanceof AvailabilityMetric)) {
    return new AvailabilityMetric()
  }

  Metric.call(this)

  this.key = 'availability'
  this.default = [0, 0] // [requests,responses]
  this.hooks = [{
    trigger: 'before',
    event: 'send',
    handler: this._start
  }, {
    trigger: 'before',
    event: 'receive',
    handler: this._stop
  }]
}
function AvailabilityMetric () {
  if (!(this instanceof AvailabilityMetric)) {
    return new AvailabilityMetric()
  }

  Metric.call(this)

  this.key = 'availability'
  this.default = [0, 0] // [requests,responses]
  this.hooks = [{
    trigger: 'before',
    event: 'send',
    handler: this._start
  }, {
    trigger: 'before',
    event: 'receive',
    handler: this._stop
  }]
}
Defines a metric that measures availability by number of requests/responses
@constructor