get_task_metrics() is expensive and must be destroyed

[divergentdave]

Here's a recently executed query plan:

[inahga]

From discussion out of band, we probably will want to delete this function as-it-is. While it has information that might be useful, it is slow for large tasks even with index scans, there may be better ways to get the same information, and it isn't currently useful for billing purposes.

[tgeoghegan]

Elsewhere we have work underway (#2293, #2508, #2537, #2553, #2513) to introduce a superior task metrics solution. So I think this issue only tracks deleting the old solution once the new one is deployed.

@inahga: besides deploying the new stuff, are there other blockers to deleting the old stuff?