collector: reduce filesystem and textfile allocations

[kevinburke]

Store filesystem read-only state during mount parsing instead of serializing and reparsing mount options on every scrape. Rework textfile metric family conversion to build one desc per family and normalize label sets by index.

Benchstat:
goos: linux
goarch: amd64
pkg: github.com/prometheus/node_exporter/collector 
cpu: AMD Ryzen 9 8945HS w/ Radeon 780M Graphics

                              │ /tmp/base.txt │          /tmp/current.txt           │
                              │    sec/op     │   sec/op     vs base                │
ParseFilesystemLabels-8          219.78µ ± 0%   98.89µ ± 1%  -55.01% (p=0.000 n=20)
TextfileConvertMetricFamily-8     466.0µ ± 1%   223.5µ ± 1%  -52.04% (p=0.000 n=20)
geomean                           320.0µ        148.7µ       -53.55%

                              │ /tmp/base.txt │           /tmp/current.txt           │
                              │     B/op      │     B/op      vs base                │
ParseFilesystemLabels-8         191.29Ki ± 0%   99.28Ki ± 0%  -48.10% (p=0.000 n=20)
TextfileConvertMetricFamily-8    472.0Ki ± 0%   292.8Ki ± 0%  -37.95% (p=0.000 n=20)
geomean                          300.5Ki        170.5Ki       -43.25%

                              │ /tmp/base.txt │          /tmp/current.txt           │
                              │   allocs/op   │  allocs/op   vs base                │
ParseFilesystemLabels-8           4.169k ± 0%   1.097k ± 0%  -73.69% (p=0.000 n=20)
TextfileConvertMetricFamily-8    13.054k ± 0%   7.955k ± 0%  -39.06% (p=0.000 n=20)
geomean                           7.377k        2.954k       -59.96%

[yuvipanda]

We're noticing that on GKE sometimes each scrape takes like, 20-30s, and readonly errors seem to be the primary difference vs our EKS clusters (which are faster). Are you experiencing something like that kinda slowdown?

[SuperQ]

Mount options can change between scrapes.

This would cache this value permanently. We need to parse every scrape to find out if someone ran mount -o remount,rw, for example.