Add WAPE metric

[alex-hse-repository]

Proposal

1. Add function wape into etna/metrics/functional_metrics.py, see wape
2. Add class WAPE into etna/metrics/metrics.py
3. Add tests like for other metrics

Discussed in #1063

^{Originally posted by prokofyev January 9, 2023}
Hello, dear developers!

I am interested why don't you have WAPE metric built-in in your ETNA library? I believe it would be highly popular and used.

Many thanks!

[looopka]

hello, i'm want do it on weekend)

[alex-hse-repository]

@looopka Hi! Sounds great, feel free to ask any questions)
May you also assign yourself to this issue pls

[looopka]

I can't assign this issue to myself :(.
last time, iKintosh assigned task to me.
if it's not difficult for you, assign it and #846 for me