chore(parsers.xpath): Add tracing mode to output empty-node-selection messages (#14939)

plugins/parsers/xpath/README.md

@@ -135,6 +135,10 @@ XPath expressions.
   ## Currently, CBOR, protobuf, msgpack and JSON support native data-types.
   # xpath_native_types = false
 
+  ## Trace empty node selections for debugging
+  ## This will only produce output in debugging mode.
+  # xpath_trace = false
+
   ## Multiple parsing sections are allowed
   [[inputs.file.xpath]]
     ## Optional: XPath-query to select a subset of nodes from the XML document.

plugins/parsers/xpath/parser.go

@@ -43,6 +43,7 @@ type Parser struct {
 	PrintDocument       bool              `toml:"xpath_print_document"`
 	AllowEmptySelection bool              `toml:"xpath_allow_empty_selection"`
 	NativeTypes         bool              `toml:"xpath_native_types"`
+	Trace               bool              `toml:"xpath_trace"`
 	Configs             []Config          `toml:"xpath"`
 	DefaultMetricName   string            `toml:"-"`
 	DefaultTags         map[string]string `toml:"-"`
@@ -623,7 +624,7 @@ func (p *Parser) constructFieldName(root, node dataNode, name string, expand boo
 }
 
 func (p *Parser) debugEmptyQuery(operation string, root dataNode, initialquery string) {
-	if p.Log == nil {
+	if p.Log == nil || !p.Trace {
 		return
 	}