Remove deprecated --metric-count in 0.7; update docs.

[bwplotka]

Related discussion: https://github.com/prometheus-community/avalanche/pull/80#discussion_r1761695654

In https://github.com/prometheus-community/avalanche/pull/80 we added more flags per type so users should use that ones, and we deprecate --metric-count in 0.6. Let's be nice and remove it only in the next minor release and update the documentation slowly.