RFC: expose channels_sv2::vardiff state via stratum-apps::monitoring HTTP APs

[plebhash]

as we gather community feedback, one thing is becoming clear:

vardiff UX is a bit confusing

• Vardif needs better explanation, and to show progress in the UI, until Vardiff kicks in, users are reporting 'stats are off" type of issues. sv2-ui#42
• label hashrate as estimated with informational tooltips sv2-ui#47

I'm wondering if channels_sv2::vardiff could expose some method that would allow stratum-apps::monitoring HTTP APIs to probe whether vardiff has "converged" or not

that way, sv2-ui could provide visual cues about the degree of confidence around the displayed hashrate

[plebhash]

while inspecting channels_sv2::vardiff::classic my first instinct was to simply get stratum-apps::monitoring to call VardiffState::try_vardiff, but upon deeper reflection I don't think it would be appropriate to allow stratum-apps::monitoring to mutate the internal VardiffState

perhaps VardiffState could hold a new stable: bool (or some better name) that would behave according to the last execution of VardiffState::try_vardiff:

• if it returned Ok(Some(new_hashrate)) or Err, then it's false
• if it returned Ok(None), then it's set to true

then stratum-apps::monitoring would simply probe VardiffState::stable and sv2-ui would use that to provide visual cues on whether hashrate has "converged" or not

the only problem with this approach is that I'm not sure how long it could take for VardiffState::stable to become true

[plebhash]

alternatively, instead of a bool, VardiffState could hold a "normalized delta" of the last update

if the value is low, it indicates hashrate estimation is "high confidence"

if the value is high, it indicates hashrate estimation is "low confidence"

[GitGab19]

I think users are already used to the concept of difficulty, which is already exposed by every firmware's dashboard.

Currently we're exposing only the target through the APIs, but we should probably start exposing also the corresponding difficulty.

In this way we could show it also on our UI, making it easier for users to understand what's going on?