Add a `force` option in `PowerDistributingActor` `Request`

[ela-kotulska-frequenz]

What's needed?

As it was discussed, every component metric received from microgrid can be NaN. In that case metrics are independent from each: if one metric is NaN, then others can be correct float values.

PowerDistributingActor is ignoring components with NaN metrics, now.
But when we do gap_discharge it should replace those metrics with the latest not-NaN values and use them.

Proposed solution

1. Cache latest non-NaN value as use them if current metric is NaN and we do gap_discharge.
2. Create one more flag in the PowerDistributingActor request (src/frequenz/sdk/actor/power_distributing/request.py) : gap_discharge. Actor should set it to True, if he is doing gap_discharge. This should prevent from using cached values in the normal set_power operation. It should also simplify decision making process (less branches).

Use cases

No response

Alternatives and workarounds

No response

Additional context

No response

[leandro-lucarella-frequenz]

Shouldn't be whoever is doing the gap discharge responsible for doing this work (caching the last value, etc.)? Looks like out of the scope of the power distributing actor.

[leandro-lucarella-frequenz]

If what we need is a way to force the power distributor to apply some command to all batteries regardless of their working status, I would name that option just force. gap_discharge sounds too niche and only about one possible use case for this feature, but maybe I am mistaken.

[shsms]

I agree, gap_discharge is a peak shaving concept. Ideally it should not be mentioned anywhere in the sdk codebase or docs.