Down-weight current runs too when computing wfu/tk

Otherwise the earlier runs decay to small numbers, while the current
run becomes a huge number, and then relays with high uptime dominate
all the wfu and tk stats.

For now I'm leaving the mtbf tracking alone -- that is, mtbf is still
dominated by the uptime of the current run -- because otherwise many
relays lose their Stable flag and thus their Guard flag. It's not at all
clear that we're assigning the Stable flag in a smart way, but also it's
not clear that changing it to downweight the current run is what we want
(if we downweight the total_run_weights variable, it decays more than
it should, or if we don't, it decays less than it should).

The long term answer is that we need to use past data to assess how
various approaches predict future behavior. But I'm doing this minimal
change now since this bugfix paves the way for the HSDir assignment
changes in #19162.

Resolves #40395.
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