Per-query observability metrics

When global.enable_query_metrics is set to true, every scrape emits two additional gauges per query in the configuration:

• query_duration_seconds{query="<query_name>"} - wall-clock time the query took during the most recent scrape, including row scanning. Emitted even when the query errors, so spikes preceding a failure remain visible.
• query_rows_returned{query="<query_name>"} — number of rows the database returned during the most recent scrape. Errored or skipped rows are not counted.

Both metrics inherit the same constant labels as up / scrape_duration_seconds (notably target in multi-target / jobs mode), so they can be aggregated by target the same way. The feature is off by default to keep the existing metric surface unchanged.