Simplify prometheus_metrics and its tests

From https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8434#note_119158757

ee/app/assets/javascripts/prometheus_metrics/prometheus_metrics.js has a lot of repetitive lines for hide/show classes. This is hard to read/understand, and very error prone. Can mean we forget to hide an element sometimes, or hide everything and have a blank screen.

I suggest changing to:

  // $els is all the `.find` things at the top
  hideAll() {
    this.$els.forEach(e => e.addClass('hidden'))
  }

  setVisible(...els) {
    this.hideAll()
    els.forEach(el => el.removeClass('hidden'))  
  }

  // later in the file
  case PANEL_STATE.NO_INTEGRATION:
    this.setVisible(this.customMetricsNoIntegrationText, this.customMetricsNoIntegrationText)    
  default:
    this.setVisible(this.monitoredCustomMetricsEmpty)

Can make a similar change for the karma test.