You can now use Metrics Explorer to find individual DoFns that cause latencies in streaming jobs

## Feature You can now [use Metrics Explorer](https://cloud.google.com/dataflow/docs/guides/using-cloud-monitoring#metrics-explorer) to find individual DoFns that cause latencies in streaming jobs. These metrics are available in streaming pipelines that use Apache Beam 2.53.0 and later versions. The following new metrics are available: * Average message processing time per DoFn (`job/dofn_latency_average`) * Maximum message processing time per DoFn (`job/dofn_latency_max`) * Minimum message processing time per DoFn (`job/dofn_latency_min`) * Number of messages processed per DoFn (`job/dofn_latency_num_messages`) * Oldest active message processing time per DoFn (`job/oldest_active_message_age`) * Total message processing time per DoFn (`job/dofn_latency_total`) For more information about Dataflow metrics, see [Google Cloud metrics](https://cloud.google.com/monitoring/api/metrics%5Fgcp#gcp-dataflow).