collector: use the shared exporter-toolkit collector registry (PoC)

[nicolastakashi]

Draft / proof of concept. Stacked on prometheus/exporter-toolkit#429 — CI will not build until that lands, since the toolkit package it imports is unreleased. To build locally: go mod edit -replace github.com/prometheus/exporter-toolkit=/path/to/exporter-toolkit against that branch.

Follow-up to @ArthurSens' question in #1361: could the collector enable/disable abstraction live in exporter-toolkit, and could the docs table be generated from it? This is what that looks like in practice.

What changed

collector/collector.go's factories / collectorState / forcedCollectors / initiatedCollectors maps, and the flag registration, forced-state tracking, filtering and lazy instantiation around them, are near-verbatim copies of node_exporter's. They're replaced by toolkit.Registry, which is generic over the collector interface — so Update(ctx, instance, ch) is untouched, and registerCollector keeps its signature. None of the ~26 per-collector files needed changes for the refactor itself.

probe.go had a third partial copy of the instantiate-and-cache loop, carrying a // TODO: Handle filters that node_exporter's copy had already solved. It now shares the registry, so that divergence is gone.

Net −117/+113 including the new flags and metadata.

The docs table becomes generated

Four collectors move to RegisterCollectorWithMetadata to record operator preconditions — the pg_stat_statements and pg_buffercache extensions, minimum server versions:

$ postgres_exporter --collector.list.markdown
| Collector | Default | Notes |
|---|---|---|
| `buffercache_summary` | disabled | Shared buffer cache summary; requires the `pg_buffercache` extension, PostgreSQL 16+ |
| `database` | enabled |  |
...
| `stat_statements` | disabled | Statement execution statistics; requires the `pg_stat_statements` extension |

This reproduces all 26 rows of the hand-written table in #1361, defaults included — which is a good sign for that table, it's accurate today. The point is that it stays accurate. --collector.list emits the same inventory as JSON for mdox or a CI drift check.

Runtime introspection

pg_collector_enabled{collector="..."} is now exposed, so which collectors an exporter actually has enabled is answerable from Prometheus across a fleet, rather than only by inspecting a process's command line.

Notes

• --collector.disable-defaults comes for free from the registry (DisableDefaultCollectors is exported) but I left it unwired — adding a flag postgres_exporter doesn't have today is a UX call for maintainers, not part of a refactor.
• Verified: --no-collector.X / --collector.X behave exactly as before, and the full suite passes (107 tests).
• I'd suggest not blocking Create user facing documentation #1361 on this. Ship the hand-written table now; a slightly stale table is a much smaller problem than no docs, and this can replace it later.