swap: added cheques metrics

[holisticode]

This PR adds metrics for cheques monitoring to the existing metrics.

[mortelli]

no blockers.

maybe we can add something like a cashed cheques metric as well, no?

[holisticode]

@mortelli we could, but as currently the cheques are cashed right away, I think it may be enough for now to keep just received cheques - until we don't change cashing strategy, it should essentially be the same no?

[ralph-pichler]

@holisticode Even in the current implementation we don't cash every cheque right away, so there is already a difference. I'm not convinced that the number of cashed cheques alone is useful (except perhaps in the context of #2012, but that is different from metrics), but the total amount of honey cashed probably would be. (What does number of cashed cheques even mean in the context of cumulatives cheques? If I received 3 from the same peer and I cashed the first and the third, did I cash 2 or 3?)

[holisticode]

Ok agree. Will add a cheques.cashed.honey metric

[holisticode]

Closes #2019