Add a way to inspect metrics for each task

[jayshrivastava]

The plan for #123 is to aggregate / merge metrics for all tasks together so we have "per stage" metrics. Each node may have metrics in the form of output_rows = X, output_rows = Y, output_rows = Z where X, Y, Z are metrics for each task. The aggregation output_rows = X + Y + Z is shown in the explain analyze output.

Unfortunately, this hides metrics per task, which would be useful during debugging.

There's some options to make metrics inspectable

• add labels to metrics so they are tagged with task id
• make EXPLAIN VERBOSE display metrics per task
• add a debug log line which makes each task emit a log of its local plan with metrics