[WIP] Add a metric to track request failures

[harche]

Add a metric to track request failures

FIX #9346

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[markmc]

Couple of points:

• For new metrics, the priority should be to add them in V1 since V0 will shortly be deprecated
• Are FINISHED_ABORTED requests already counted under request_success_total[length] ... is this counter poorly named and should be request_finished_total ?
• Are failed HTTP requests already counted by the HTTP metrics middleware?