Backport latest changes from polkadot staging (#2732)

* separate constants for average and worst case relay headers (#2728)

* separate constants for average and worst case relay headers

* fix compilation

* Select header that will be fully refunded in on-demand batch finality relay (#2729)

* select header that will be fully refunded for submission in on-demand **batch** finality relay

* added the only possible test

* spelling

* nl

* updated comment

* backport some nits from https://github.com/paritytech/parity-bridges-common/pull/2727

* NotApplicable instead of Unroutable

* Grafana update stuff (#2733)

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Co-authored-by: Branislav Kontur <[email protected]>