Read current and peak queued task payload usage at dispatcher, plus the configured capacity, in MB (metric, 1 MB = 1,000,000 bytes).
.dispatcher_capacity(disp)Named numeric vector of length 3: used (current) and peak
(high-watermark) usage, and capacity (the capacity set on
.dispatcher_start(), NA_real_ if unset/unbounded), all in MB.
NA_real_ in each slot if disp is invalid.
External pointer to dispatcher handle.