[chore][internal/sqlquery] Make error public for future use (#32175)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
This is part of
#31915,
to try to make it more manageable in size. This variable will be used by
the SQL Server receiver.

internal/sqlquery/db_client_test.go

@@ -70,7 +70,7 @@ func TestDBSQLClient_Nulls(t *testing.T) {
 	}
 	rows, err := cl.QueryRows(context.Background())
 	assert.Error(t, err)
-	assert.True(t, errors.Is(err, errNullValueWarning))
+	assert.True(t, errors.Is(err, ErrNullValueWarning))
 	assert.Len(t, rows, 1)
 	assert.EqualValues(t, map[string]string{
 		"col_0": "42",
@@ -96,7 +96,7 @@ func TestDBSQLClient_Nulls_MultiRow(t *testing.T) {
 		assert.Len(t, uw, 2)
 
 		for _, err := range uw {
-			assert.True(t, errors.Is(err, errNullValueWarning))
+			assert.True(t, errors.Is(err, ErrNullValueWarning))
 		}
 	}
 	assert.Len(t, rows, 2)

internal/sqlquery/row_scanner.go

@@ -12,7 +12,7 @@ import (
 	"go.uber.org/multierr"
 )
 
-var errNullValueWarning = errors.New("NULL value")
+var ErrNullValueWarning = errors.New("NULL value")
 
 type rowScanner struct {
 	cols       map[string]func() (string, error)
@@ -28,7 +28,7 @@ func newRowScanner(colTypes []colType) *rowScanner {
 		var v any
 		rs.cols[colName] = func() (string, error) {
 			if v == nil {
-				return "", errNullValueWarning
+				return "", ErrNullValueWarning
 			}
 			format := "%v"
 			if t, isTime := v.(time.Time); isTime {

internal/sqlquery/scraper.go

@@ -71,7 +71,7 @@ func (s *Scraper) Scrape(ctx context.Context) (pmetric.Metrics, error) {
 	out := pmetric.NewMetrics()
 	rows, err := s.Client.QueryRows(ctx)
 	if err != nil {
-		if errors.Is(err, errNullValueWarning) {
+		if errors.Is(err, ErrNullValueWarning) {
 			s.Logger.Warn("problems encountered getting metric rows", zap.Error(err))
 		} else {
 			return out, fmt.Errorf("Scraper: %w", err)