Add Head.AppendableMinValidTime() (#9643)
Add Head.AppendableMinValidTime() (#9643)

Signed-off-by: Marco Pracucci <3829486b93ec44395f0b980424bae9b6fb07b7bc@pracucci.com>
keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus