Add per-task run time metric

[b-deam]

Current we collect service_time, processing_time and latency summary metrics for all tasks by default. It would also be useful if we collected a total time taken to execute (run_time, execution_time, total_time ?) metric for the task object as a whole.

This new summary metric that reports the total time taken per task would allow users to sum them to get the total runtime of any combination of tasks.