Custom callback functions to dynamically create eventLabel from vjs object. Also custom callback to generate custom dimension/metrics. The play event is actually somewhat unreliable, so we'll switch it to playing. This will assure the event is only fired after canplay on the source has been assured. loadstart event to reliably generate labels and metrics, rather than loadedmetadata. To reliable assure we have eventLabel and custom metrics and dimensions, these will be generated on playing event too.