hwmon: disambiguate colliding chip labels

[mwimpelberg28]

Summary

Fixes #3637.

Multiple hwmon nodes can be registered under a single parent device — for example, asus-nb-wmi on recent ASUS laptops registers one hwmon for fan control and another for WMI sensors. Both device symlinks resolve to the same /sys/devices/platform/asus-nb-wmi, so hwmonName produces the same platform_asus_nb_wmi chip label for both, and any sensor file that exists in both nodes (e.g. pwm1_enable) trips:

collected metric "node_hwmon_pwm_enable" { ... chip="platform_asus_nb_wmi" ... } was collected before with the same name and label values

Approach

Update now does two passes:

• Pass 1: enumerate /sys/class/hwmon/*, compute the device-derived base chip name for each, and count collisions.
• Pass 2: when a base name is shared, suffix the chip label with the chip's name file content if it disambiguates, otherwise with the hwmonX basename (always unique within a boot). The include/exclude filter is also moved here so user regexes match the label that is actually emitted in the metric.

Entries that already produce a unique chip label are unaffected — no surprise suffixes for users not hitting the collision.

This is closer in spirit to the discussion in #333 (the same class of bug for dual-socket coretemp boxes), but contained: the fix only kicks in when an actual collision is detected.

Test plan

New collector/hwmon_linux_test.go:

• TestHwmonDuplicateChipNamesAreDisambiguated — reproduces the Metric node_hwmon_pwm_enable was collected before with the same name and label values #3637 ASUS WMI scenario (two hwmon dirs sharing one platform device, both exposing pwm1_enable) and asserts both Gather succeeds and the chip labels are distinct.
• TestHwmonUniqueChipNamesAreUnchanged — guards against unintended label drift for users not hitting the collision.
• TestHwmonDuplicateChipNamesWithSameNameFile — exercises the hwmonX-basename fallback when the name file content also collides.
• Full collector test suite still passes (go test ./collector/), including the existing fixture-driven e2e checks.
• go vet ./... clean.

[mwimpelberg28]

@SuperQ would you have a moment to take a look? Happy to address any feedback.