RFE: collect Linux 'irq' pressure information when available

[siebenmann]

Linux kernels since 6.1 that are built with CONFIG_IRQ_TIME_ACCOUNTING support a new /proc/pressure/irq pressure stall information file. It would be nice if the node exporter provided metrics from this (well, a single metric, it apparently only has the 'full' line). The complication will be that this file isn't necessarily available in all supported environments, even though the other /proc/pressure information is.