[Accumulators] How to extract P90, P99?

From: Dominique Devienne (ddevienne_at_[hidden])
Date: 2020-12-17 15:51:03

I'm already using the accumulator below to keep track of request processing
times of a server.

    using ProcAcc = acc::accumulator_set<
        double, acc::stats<
            acc::tag::min,
            acc::tag::max,
            acc::tag::mean,
            acc::tag::median,
            acc::tag::sum
        >
    >;

But now I'm interested in also extracting the P90 and P99 values,
to get insight in how much of an outlier the max could be.

AFAIK, the median is the P50, but it's not readily obvious to me what tag
to use for the P90 or P(N) in general. Could someone help please? Thanks,
--DD

PS: Does accumulator allow to keep the top(N) values as well? How?