Right now there is a single metric for speculative retry on reads that is the number of speculative retries attempted. You can't tell how many of those actually succeeded in salvaging the read.
The metric is also per table and there is no keyspace level rollup as there is for several other metrics.
Add a metric that counts reads that attempt to speculate but fail to complete before the timeout (ignoring read errors).
Add a rollup metric for the current count of speculation attempts as well as the count of failed speculations.