Believe me. I understand.
There are some technical reasons why a metric aggregated at the host has difficulty with percentile and maybe why NR has chosen at this time not to include it. Just totally guessing. They could also be short staffed or have other priorities.
With an event you have all the data on the NR side and each event is more often a representation of a single transaction with it’s timing. You then query that data and aggregate in realtime using NRQL etc. You have total control with event data and its accurate*
Metric data is aggregated at the agent or host most of the time and you do not have per transaction data or attributes to query. Because the monitors are distributed you have some challenges. I’m maybe less understanding of the lack of percentile data for any metric derived from events that are still present in NR.
For percentiles that might be difficult to aggregate across thousands of hosts. You need to approximate, use math, some voodoo or witchcraft.
I was going to go into more detail, but I decided I could maybe find an article that explains it better than I can. The code is there. Maybe NR can give it a whirl.