Smarter queue ETAs using request metadata

The queue already shows ETAs and queue position. We could make ETAs more accurate by using more metadata than the average of past run durations.

Ideas

• Condition on input size or parameters (e.g. number of steps, resolution, audio length)
• Take the hardware and the current concurrency into account
• Allow developers to supply an ETA hint from their function

Also make sure computing ETAs adds no noticeable overhead to the queue.