• Added a required parameter screen to the Event.devtoolsEvent constructor.
• Added an optional parameter additionalMetrics to the Event.devtoolsEvent constructor.
• Added CustomMetrics class for unified_analytics clients to define custom event metrics.
• Removed parameters uiDurationMicros, rasterDurationMicros, shaderCompilationDurationMicros,
  traceEventCount, cpuSampleCount, cpuStackDepth, heapDiffObjectsBefore, heapDiffObjectsAfter,
  heapObjectsTotal, rootSetCount, rowCount, inspectorTreeControllerId, androidAppId, iosBundleId
  from the Event.devtoolsEvent constructor.