fix: handle NULL pg_stat_wal.stats_reset (minimal)

[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 — minimal version

This is the minimal version of the fix: just the string → sql.NullString
type change in PgStatWal and the corresponding .String accessor at the
metric-label call sites. No refactor, no new tests; relies on the existing
E2E suite for integration coverage. Net diff: 2 files, 9 lines.

A larger version that also extracts a getPgStatWAL(db, version) helper and
adds three sqlmock-backed unit tests (populated / NULL on PG<18 / NULL on
PG≥18) is available on branch dev/9106 (see fork PR #1). Reviewer
preference decides which scope ships.

Testing

• Build clean: go build ./...
• Vet clean on touched packages
• 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 and design choices were
reviewed by the author before submission.