[pkg/stanza] Enhance error message in csv_parser (#13432)

When this error is triggered, it is very helpful to know what was
expected and found.

pkg/stanza/operator/parser/csv/csv.go

@@ -196,7 +196,7 @@ func generateParseFunc(headers []string, fieldDelimiter rune, lazyQuotes bool) p
 		parsedValues := make(map[string]interface{})
 
 		if len(joinedLine) != len(headers) {
-			return nil, errors.New("wrong number of fields")
+			return nil, fmt.Errorf("wrong number of fields: expected %d, found %d", len(headers), len(joinedLine))
 		}
 
 		for i, val := range joinedLine {

pkg/stanza/operator/parser/csv/csv_test.go

@@ -65,14 +65,14 @@ func TestParserByteFailure(t *testing.T) {
 	parser := newTestParser(t)
 	_, err := parser.parse([]byte("invalid"))
 	require.Error(t, err)
-	require.Contains(t, err.Error(), "wrong number of fields")
+	require.Contains(t, err.Error(), "wrong number of fields: expected 3, found 1")
 }
 
 func TestParserStringFailure(t *testing.T) {
 	parser := newTestParser(t)
 	_, err := parser.parse("invalid")
 	require.Error(t, err)
-	require.Contains(t, err.Error(), "wrong number of fields")
+	require.Contains(t, err.Error(), "wrong number of fields: expected 3, found 1")
 }
 
 func TestParserInvalidType(t *testing.T) {

unreleased/pkg-stanza-csv-err.yaml

@@ -0,0 +1,16 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: "`filelog`, `journald`, `syslog`, `tcplog`, `udplog`, `windowseventlog` receivers"
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Enhance error message when csv_parser finds unexpected number of fields
+
+# One or more tracking issues related to the change
+issues: [13427]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: