collector/slabinfo: index duplicate slab entries

[machismo0311]

Fixes #3506.

/proc/slabinfo may contain repeated slab names, for example one cache per
device instance. The collector currently labels series only by slab name, so
duplicates produce identical label sets and cause Gather() to fail for the
entire scrape.

This change adds an index label, following the direction agreed in the issue,
so every cache keeps its own series, counts, and geometry.

Notes for review:

• index is assigned after the include/exclude filter, so an excluded slab does
  not leave a gap in the ordinals.
• The ordinal is positional and is not a stable identity. If a cache appears or
  disappears, subsequent indices can shift.
• Geometry remains per entry. The kernel does not guarantee that same-named
  caches share object_size, so the exporter does not choose a representative
  value.
• This collector previously had no dedicated test file. Four tests are added
  covering duplicate names, distinct names, differing geometry, and the
  filter/index interaction.
• Both e2e golden files are updated. Values are unchanged. Only the new label is
  added.

Validated with:

• gofmt
• go vet ./...
• go test -short ./...
• golangci-lint using the repository-pinned version
• amd64, 386, and arm64 builds
• exact comparison of slabinfo fixture output

[nicolastakashi]

collector/slabinfo_linux_test.go:1 uses // Copyright 2026 The Prometheus Authors. New files need the year-less form, // Copyright The Prometheus Authors — make check_license rejects 2026 or later.

Everything else looks good, including the index label: it matches the precedent already shipped in rapl_linux.go and sysctl_linux.go (both use a bare index label alongside a descriptive companion label), so no rename needed there.

[nicolastakashi]

/workflow-approve

[machismo0311]

Fixed, thanks. The test file now uses the year-less header; check_license and the future-year grep in Makefile.common both pass locally. Amended in place, no other changes.

[nicolastakashi]

/workflow-approve