stats: show current calculated day as dotted point

[sevenzing]

For now, the stats API removes the last point from the response if the value was calculated today::
https://github.com/blockscout/blockscout-rs/blob/main/stats/stats-server/src/read_service.rs#L98

We actually want to display this point, but add a note to inform the user that this value is partially calculated. For example, from Mixpanel::

This feature requires API changes, so one should discuss with the frontend team the best way to implement it

[bragov4ik]

i think it makes sense to link pr #836
👀

[bragov4ik]

This feature is deployed in backend. There are some issues (probably with update logic) that result in too inaccurate approximate points, but this should not be a problem in v2 due to large refactor.