WIP - Implement quantile summary metric

[jonasrichard]

Bumping versions (prometheus.erl was around 4.2 so it is 4.8.x now).

Inconsistency in prometheus.erl because observe-time quantiles are in nanosecond not second. I fixed in the test but it is clearly a downstream bug.

  1) test observe_duration block (Prometheus.QuantileSummaryTest)
     test/metric/quantile_summary_test.exs:189
     ** (MatchError) no match of right hand side value: {2, 1.00099, [{0.5, 1000988000}, {0.9, 1000988000}, {0.95, 1000988000}]}
     code: {count, sum} = QuantileSummary.value(spec)
     stacktrace:
       test/metric/quantile_summary_test.exs:212: (test)

[jonasrichard]

I am using my own fork until the change in prometheus.erl is not merged.