log queries and record metric where columns are null

Author: a-monteiro

query.go

@@ -159,7 +159,7 @@ func (q *Query) Collect(ctx context.Context, conn *sql.DB, ch chan<- Metric) {
 			}
 
 			// Apply lag calculations and other transformations
-			transformedRow := q.applyTransformations(row, mf.config)
+			transformedRow := q.applyTransformations(row, mf.config, ch)
 
 			mf.Collect(transformedRow, ch)
 			metricsGenerated++
@@ -416,7 +416,7 @@ func (q *Query) columnIsNeeded(column string, metric *config.MetricConfig) bool
 }
 
 // applyTransformations applies configured transformations like lag calculations to a row
-func (q *Query) applyTransformations(row map[string]any, metric *config.MetricConfig) map[string]any {
+func (q *Query) applyTransformations(row map[string]any, metric *config.MetricConfig, ch chan<- Metric) map[string]any {
 	result := make(map[string]any)
 
 	// Check for NULL values in columns we actually care about (after row filtering)
@@ -445,6 +445,8 @@ func (q *Query) applyTransformations(row map[string]any, metric *config.MetricCo
 	// Log NULL columns only for rows that passed filtering and columns we need
 	if len(nullColumns) > 0 {
 		slog.Warn("Key columns are NULL", "logContext", q.logContext, "columns", nullColumns, "table_column", tableColumn, "sql", q.config.Query)
+		// Also register this as a scrape error
+		ch <- NewInvalidMetric(errors.Errorf(q.logContext, "key columns are NULL: %v (table_column: %s)", nullColumns, tableColumn))
 	}
 
 	// Copy original row data