Add optional no-query-text pg_stat_statements collectors

[maltere]

Description

This adds optional postgres_exporter collector files that retrieve pg_stat_statements metrics without loading retained query text.

PostgreSQL loads its complete query-text storage before evaluating expressions such as left(s.query, 40). Large retained statements can therefore make the existing collectors expensive and cause scrape timeouts.

The new collectors:

• use public.pg_stat_statements(false);
• preserve existing metric names and labels;
• emit an empty query label;
• continue exporting queryid, role, database, timing, row and WAL metrics.

Existing collectors remain unchanged, preserving current dashboard behavior. Documentation explains how to select the no-query-text variant and its observability trade-off.

• Bugfix
• Enhancement
• Breaking Change
• Documentation

closes #489

Testing

• Installation method:
  • Other: source-tree collector validation
• PostgreSQL: 13, 14, 15, 16 and 17
• Hugo 0.166

Code testing

• Executed all five rendered collector queries against PostgreSQL 13–17.
• Verified pg_stat_statements(false) returns statistics without query text using retained large utility statements.
• Confirmed collector names and metric schemas match the existing collectors.
• Parsed all collector and changelog files as YAML.
• Built the documentation successfully.
• Ran the changed-file pre-commit suite.

Checklist

• Documentation updated
• Release notes updated
• Upgrade documentation not required