[pkg/ottl] Add grammar utility to extract paths from statements (open…

…-telemetry#35174)

**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 PR is part of
open-telemetry#29017,
and adds a grammar utility to allow extracting `ottl.path` from parsed
statements and expressions. The new functions will power the context
inferrer utility (see
[draft](open-telemetry#35050)),
which implementation will basically gather all declared statement's
`Path.Context`, and choose the highest priority one.

For the statements rewriter utility purpose, it changes the `grammar.go`
file including a new field `Pos lexer.Position` into the `ottl.path`
struct. This value is automatically set by the `participle` lexer and
holds the path's offsets, being useful for identifying their positions
in the raw statement, without using regexes an being coupled to the
grammar's definition.

**Additional changes**:

This proposal uses the visitor pattern to gather all path's from the
parsed statements. Considering the grammar's custom error mechanism
(`checkForCustomError`) also works with a similar pattern, I've added
the visitor implementation as part of the grammar objects, and
refactored all `checkForCustomError` functions to be part of a validator
visitor. Differently of the current implementation, it joins and
displays all errors at once instead of one by one, IMO, it's specially
useful for statements with more than one error, for example: `set(name,
int(2), float(1))`.

*&ast;* We can change it back do be error-by-error if necessary.
*&ast;* If modifying the custom validator is not desired, I can roll
those changes back keeping only the necessary code to extract the path's
+ `Pos` field.

**Link to tracking Issue:**
open-telemetry#29017

**Testing:** Unit tests were added

**Documentation:** No changes

pkg/ottl/grammar.go

@@ -57,6 +57,18 @@ func (b *booleanValue) checkForCustomError() error {
 	return nil
 }
 
+func (b *booleanValue) accept(v grammarVisitor) {
+	if b.Comparison != nil {
+		b.Comparison.accept(v)
+	}
+	if b.ConstExpr != nil && b.ConstExpr.Converter != nil {
+		b.ConstExpr.Converter.accept(v)
+	}
+	if b.SubExpr != nil {
+		b.SubExpr.accept(v)
+	}
+}
+
 // opAndBooleanValue represents the right side of an AND boolean expression.
 type opAndBooleanValue struct {
 	Operator string        `parser:"@OpAnd"`
@@ -67,6 +79,12 @@ func (b *opAndBooleanValue) checkForCustomError() error {
 	return b.Value.checkForCustomError()
 }
 
+func (b *opAndBooleanValue) accept(v grammarVisitor) {
+	if b.Value != nil {
+		b.Value.accept(v)
+	}
+}
+
 // term represents an arbitrary number of boolean values joined by AND.
 type term struct {
 	Left  *booleanValue        `parser:"@@"`
@@ -87,6 +105,17 @@ func (b *term) checkForCustomError() error {
 	return nil
 }
 
+func (b *term) accept(v grammarVisitor) {
+	if b.Left != nil {
+		b.Left.accept(v)
+	}
+	for _, r := range b.Right {
+		if r != nil {
+			r.accept(v)
+		}
+	}
+}
+
 // opOrTerm represents the right side of an OR boolean expression.
 type opOrTerm struct {
 	Operator string `parser:"@OpOr"`
@@ -97,6 +126,12 @@ func (b *opOrTerm) checkForCustomError() error {
 	return b.Term.checkForCustomError()
 }
 
+func (b *opOrTerm) accept(v grammarVisitor) {
+	if b.Term != nil {
+		b.Term.accept(v)
+	}
+}
+
 // booleanExpression represents a true/false decision expressed
 // as an arbitrary number of terms separated by OR.
 type booleanExpression struct {
@@ -118,6 +153,17 @@ func (b *booleanExpression) checkForCustomError() error {
 	return nil
 }
 
+func (b *booleanExpression) accept(v grammarVisitor) {
+	if b.Left != nil {
+		b.Left.accept(v)
+	}
+	for _, r := range b.Right {
+		if r != nil {
+			r.accept(v)
+		}
+	}
+}
+
 // compareOp is the type of a comparison operator.
 type compareOp int
 
@@ -187,6 +233,11 @@ func (c *comparison) checkForCustomError() error {
 	return err
 }
 
+func (c *comparison) accept(v grammarVisitor) {
+	c.Left.accept(v)
+	c.Right.accept(v)
+}
+
 // editor represents the function call of a statement.
 type editor struct {
 	Function  string     `parser:"@(Lowercase(Uppercase | Lowercase)*)"`
@@ -210,13 +261,28 @@ func (i *editor) checkForCustomError() error {
 	return nil
 }
 
+func (i *editor) accept(v grammarVisitor) {
+	v.visitEditor(i)
+	for _, arg := range i.Arguments {
+		arg.accept(v)
+	}
+}
+
 // converter represents a converter function call.
 type converter struct {
 	Function  string     `parser:"@(Uppercase(Uppercase | Lowercase)*)"`
 	Arguments []argument `parser:"'(' ( @@ ( ',' @@ )* )? ')'"`
 	Keys      []key      `parser:"( @@ )*"`
 }
 
+func (c *converter) accept(v grammarVisitor) {
+	if c.Arguments != nil {
+		for _, a := range c.Arguments {
+			a.accept(v)
+		}
+	}
+}
+
 type argument struct {
 	Name         string  `parser:"(@(Lowercase(Uppercase | Lowercase)*) Equal)?"`
 	Value        value   `parser:"( @@"`
@@ -227,6 +293,10 @@ func (a *argument) checkForCustomError() error {
 	return a.Value.checkForCustomError()
 }
 
+func (a *argument) accept(v grammarVisitor) {
+	a.Value.accept(v)
+}
+
 // value represents a part of a parsed statement which is resolved to a value of some sort. This can be a telemetry path
 // mathExpression, function call, or literal.
 type value struct {
@@ -251,8 +321,27 @@ func (v *value) checkForCustomError() error {
 	return nil
 }
 
+func (v *value) accept(vis grammarVisitor) {
+	vis.visitValue(v)
+	if v.Literal != nil {
+		v.Literal.accept(vis)
+	}
+	if v.MathExpression != nil {
+		v.MathExpression.accept(vis)
+	}
+	if v.Map != nil {
+		v.Map.accept(vis)
+	}
+	if v.List != nil {
+		for _, i := range v.List.Values {
+			i.accept(vis)
+		}
+	}
+}
+
 // path represents a telemetry path mathExpression.
 type path struct {
+	Pos     lexer.Position
 	Context string  `parser:"(@Lowercase '.')?"`
 	Fields  []field `parser:"@@ ( '.' @@ )*"`
 }
@@ -276,6 +365,14 @@ type mapValue struct {
 	Values []mapItem `parser:"'{' (@@ ','?)* '}'"`
 }
 
+func (m *mapValue) accept(v grammarVisitor) {
+	for _, i := range m.Values {
+		if i.Value != nil {
+			i.Value.accept(v)
+		}
+	}
+}
+
 type mapItem struct {
 	Key   *string `parser:"@String ':'"`
 	Value *value  `parser:"@@"`
@@ -326,6 +423,19 @@ func (m *mathExprLiteral) checkForCustomError() error {
 	return nil
 }
 
+func (m *mathExprLiteral) accept(v grammarVisitor) {
+	v.visitMathExprLiteral(m)
+	if m.Path != nil {
+		v.visitPath(m.Path)
+	}
+	if m.Editor != nil {
+		m.Editor.accept(v)
+	}
+	if m.Converter != nil {
+		m.Converter.accept(v)
+	}
+}
+
 type mathValue struct {
 	Literal       *mathExprLiteral `parser:"( @@"`
 	SubExpression *mathExpression  `parser:"| '(' @@ ')' )"`
@@ -338,6 +448,15 @@ func (m *mathValue) checkForCustomError() error {
 	return m.SubExpression.checkForCustomError()
 }
 
+func (m *mathValue) accept(v grammarVisitor) {
+	if m.Literal != nil {
+		m.Literal.accept(v)
+	}
+	if m.SubExpression != nil {
+		m.SubExpression.accept(v)
+	}
+}
+
 type opMultDivValue struct {
 	Operator mathOp     `parser:"@OpMultDiv"`
 	Value    *mathValue `parser:"@@"`
@@ -347,6 +466,12 @@ func (m *opMultDivValue) checkForCustomError() error {
 	return m.Value.checkForCustomError()
 }
 
+func (m *opMultDivValue) accept(v grammarVisitor) {
+	if m.Value != nil {
+		m.Value.accept(v)
+	}
+}
+
 type addSubTerm struct {
 	Left  *mathValue        `parser:"@@"`
 	Right []*opMultDivValue `parser:"@@*"`
@@ -366,6 +491,17 @@ func (m *addSubTerm) checkForCustomError() error {
 	return nil
 }
 
+func (m *addSubTerm) accept(v grammarVisitor) {
+	if m.Left != nil {
+		m.Left.accept(v)
+	}
+	for _, r := range m.Right {
+		if r != nil {
+			r.accept(v)
+		}
+	}
+}
+
 type opAddSubTerm struct {
 	Operator mathOp      `parser:"@OpAddSub"`
 	Term     *addSubTerm `parser:"@@"`
@@ -375,6 +511,12 @@ func (m *opAddSubTerm) checkForCustomError() error {
 	return m.Term.checkForCustomError()
 }
 
+func (m *opAddSubTerm) accept(v grammarVisitor) {
+	if m.Term != nil {
+		m.Term.accept(v)
+	}
+}
+
 type mathExpression struct {
 	Left  *addSubTerm     `parser:"@@"`
 	Right []*opAddSubTerm `parser:"@@*"`
@@ -394,6 +536,19 @@ func (m *mathExpression) checkForCustomError() error {
 	return nil
 }
 
+func (m *mathExpression) accept(v grammarVisitor) {
+	if m.Left != nil {
+		m.Left.accept(v)
+	}
+	if m.Right != nil {
+		for _, r := range m.Right {
+			if r != nil {
+				r.accept(v)
+			}
+		}
+	}
+}
+
 type mathOp int
 
 const (
@@ -464,3 +619,11 @@ func buildLexer() *lexer.StatefulDefinition {
 		{Name: "whitespace", Pattern: `\s+`},
 	})
 }
+
+// grammarVisitor allows accessing the grammar AST nodes using the visitor pattern.
+type grammarVisitor interface {
+	visitPath(v *path)
+	visitEditor(v *editor)
+	visitValue(v *value)
+	visitMathExprLiteral(v *mathExprLiteral)
+}