fix: handle NULL pg_stat_wal.stats_reset

[honi-at-simspace]

Description

pg_stat_wal.stats_reset is per-instance and can legitimately be NULL on:

• replicas (which never write WAL stats locally), and
• primaries promoted from a replica that was bootstrapped via pg_basebackup —
  the WAL stats subsystem on a standby never initialises stats_reset, and
  promotion does not reset it. After ≥1 failover (rolling minor upgrades,
  node maintenance, etc.) every pod in the cluster reaches this state.

The exporter scanned the column into a string, so on such instances every
collection failed with sql: Scan error on column index 4, name "stats_reset": converting NULL to string is unsupported. All pg_stat_wal counters were
dropped for the affected pod, and the error was logged at every scrape.

This change scans stats_reset into a sql.NullString and uses the empty
string as the metric label when NULL. The actual WAL counters
(wal_records, wal_fpi, wal_bytes, wal_buffers_full, plus the PG<18
write/sync ones) are then collected normally.

Relation to cloudnative-pg#9788

cloudnative-pg#9788 takes the orthogonal approach of skipping pg_stat_wal collection on
replicas entirely. That is correct on its own merits, but does not cover the
primary-with-NULL-stats_reset case (clusters that have failed over at least
once), which is what we observe in our environments. The two changes are
complementary — cloudnative-pg#9788 stops collecting data that isn't meaningful on replicas;
this change makes the scan robust to NULL on the primary.

Diff scope — two options for reviewers

Happy to ship either of the following; reviewer preference decides.

Option A (current PR, ~110 LOC): type-safe scan + a small refactor that
extracts the scan logic into a package-private getPgStatWAL(db, version)
helper, plus three sqlmock-backed unit tests covering populated / NULL on
PG<18 / NULL on PG≥18. Pro: regression coverage. Con: introduces a small
refactor that is not strictly required by the bug fix.

Option B (~9 LOC, no test): just change StatsReset string →
sql.NullString in PgStatWal and use .String at the metric-label call
sites. No helper, no new tests; relies on the existing E2E suite for
integration coverage. Pro: minimal review surface. Con: no unit test of the
NULL path.

If you prefer Option B I'll force-push the branch with the helper and the
new tests removed.

Testing

• Build clean: go build ./...
• Vet clean on touched packages
• New unit tests in probes_test.go cover three cases: populated stats_reset
  on PG<18, NULL stats_reset on PG<18, NULL stats_reset on PG≥18.
• go test ./pkg/management/postgres/... ./pkg/management/postgres/webserver/metricserver/... passes.

Closes cloudnative-pg#9106

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Drafted with AI assistance (Claude). All code, tests, and design choices were
reviewed by the author before submission.