IP socket options (TTL / hop-limit / DSCP) — depends on prometheus/exporter-toolkit#396

[randomizedcoder]

Self-review PR — opened against randomizedcoder/node_exporter:master so the diff can be reviewed in the GitHub UI before opening upstream. Depends on the toolkit-side PR landing first.

What this picks up

The IP socket-options feature being added to prometheus/exporter-toolkit in prometheus/exporter-toolkit#396. Once that PR merges and the toolkit cuts a new release, node_exporter just needs to bump its exporter-toolkit dependency and the three new flags appear in --help automatically — no node_exporter code change beyond go.mod and the README.

New surface visible to a node_exporter operator:

• --web.ipv4-ttl / WEB_IPV4_TTL / ip_socket_config.ipv4_ttl (range 1–255)
• --web.ipv6-hop-limit / WEB_IPV6_HOP_LIMIT / ip_socket_config.ipv6_hop_limit (range 1–255)
• --web.dscp / WEB_DSCP / ip_socket_config.dscp (range 0–63)

Changes in this PR

File	Change
docs/IP_SOCKET_CONFIG.md	Already on this branch from the design phase. Canonical spec for the feature (intro, ToC, per-step DoD, summary).
README.md	New section "IP socket options" with operator-facing examples and a link to docs/IP_SOCKET_CONFIG.md.
go.mod, go.sum	Temporary replace github.com/prometheus/exporter-toolkit => github.com/randomizedcoder/exporter-toolkit @master so this PR is buildable + testable against the unmerged toolkit changes. Will be removed before opening the upstream PR and replaced with a require of a real released version.

Why a replace directive

The toolkit-side PR (prometheus#396 upstream) isn't merged or tagged yet. To make this PR diff reviewable on GitHub and go build runnable, we need to point exporter-toolkit at a place where the changes actually live — the fork's master, which now has the merged feature commits (via #1 on randomizedcoder/exporter-toolkit).

Before opening the actual upstream prometheus/node_exporter PR, the workflow is:

1. Wait for prometheus/exporter-toolkit#396 to merge.
2. Wait for a new exporter-toolkit release tag containing those commits.
3. cd node_exporter && go mod edit -dropreplace github.com/prometheus/exporter-toolkit && go get github.com/prometheus/exporter-toolkit@<new-tag> && go mod tidy.
4. Amend this commit, push, and open upstream.

Test plan

• go build ./... clean against the fork-resolved toolkit.
• ./node_exporter --help shows all three new flags with their env-var bindings.
• Smoke tests against a running exporter (see toolkit PR Don't leak or race in FreeBSD devstat collector prometheus/node_exporter#396 for the full end-to-end verification — all four config paths (flag/env/YAML/precedence) plus both validation paths confirmed).

Design doc

docs/IP_SOCKET_CONFIG.md is the canonical spec — security rationale, configuration surface, step-by-step plan with Definition of Done per step, platform support matrix, and a companion-doc link to the toolkit-side brief.

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