df.list_instances() had no way to filter by label server-side — callers had to post-filter with a WHERE clause.

Changes

• src/monitoring.rs: Added filter_label TEXT DEFAULT NULL as a third parameter; SPI query now covers all four combinations of status_filter × filter_label with parameterized queries
• sql/pg_durable--0.2.2--0.2.3.sql: Migration drops old 2-arg signature, creates new 3-arg one
• sql/pg_durable--0.1.1.sql: Updated pgrx-generated baseline to match new signature
• Cargo.toml: Version bump 0.2.2 → 0.2.3
• USER_GUIDE.md: Added filter_label examples; updated GRANT to 3-arg signature
• tests/e2e/sql/05_monitoring_and_explain.sql: Tests matching label, non-matching label (empty result), and combined status + label filter

Usage

-- Filter by label
SELECT * FROM df.list_instances(filter_label => 'my-job');

-- Filter by status and label
SELECT * FROM df.list_instances('completed', filter_label => 'my-job');