fix: use avg instead of sum for join actor metrics

[lmatz]

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What's changed and what's your intention?

fix #14448 (comment)

summing over all the actors that belong to the same fragment makes little sense, and it even sums three different sides, i.e. left, right and barrier.

avg should make more sense than sum.

Does quantile, i.e. 50th, 90th, 99th, make even more sense than avg?

Previously

Now

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