monitoring: Use overall transcode latency in Grafana dashboard

[yondonfu]

Currently, the Grafana dashboard uses the livepeer_transcode_latency_seconds metric in latency focused graphs. This metric represents the transcode latency from the time the segment is produced from the segmenter until one of the transcoded renditions is received. This metric does not capture latency induced by additional steps after a rendition is received (i.e. waiting for all renditions to be received before playlist insertion and verification when running in on-chain mode). I think using the livepeer_overall_transcode_latency_seconds metric would capture this information.