[pkg/ottl] Add ability to parse conditions (open-telemetry#29315)

**Description:** 
This PR adds a new public API to support parsing and using conditions. 

This implementation opted to add ParseConditions directly to the
existing Parser instead of creating a second Parser struct. Since any
Parser needs the Context, Functions, PathExpressionParser, EnumParser,
and telemetry settings the 2 structs would be incredibly similar.

See these structs implemented in components here:
open-telemetry#29294

**Link to tracking Issue:** <Issue number if applicable>
Works towards
open-telemetry#13545

**Testing:** <Describe what testing was performed and which tests were
added.>
Added tests

**Documentation:** <Describe the documentation added.>
Added godoc strings

.chloggen/ottl-condition-parser-add-interfaces.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# 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: pkg/ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add ability to independently parse OTTL conditions.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [29315]
+
+# (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:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [api]

pkg/ottl/parser.go

@@ -32,13 +32,6 @@ func (e *ErrorMode) UnmarshalText(text []byte) error {
 	}
 }
 
-type Parser[K any] struct {
-	functions         map[string]Factory[K]
-	pathParser        PathExpressionParser[K]
-	enumParser        EnumParser
-	telemetrySettings component.TelemetrySettings
-}
-
 // Statement holds a top level Statement for processing telemetry data. A Statement is a combination of a function
 // invocation and the boolean expression to match telemetry for invoking the function.
 type Statement[K any] struct {
@@ -66,6 +59,26 @@ func (s *Statement[K]) Execute(ctx context.Context, tCtx K) (any, bool, error) {
 	return result, condition, nil
 }
 
+// Condition holds a top level Condition. A Condition is a boolean expression to match telemetry.
+type Condition[K any] struct {
+	condition BoolExpr[K]
+	origText  string
+}
+
+// Eval returns true if the condition was met for the given TransformContext and false otherwise.
+func (c *Condition[K]) Eval(ctx context.Context, tCtx K) (bool, error) {
+	return c.condition.Eval(ctx, tCtx)
+}
+
+// Parser provides the means to parse OTTL Statements and Conditions given a specific set of functions,
+// a PathExpressionParser, and an EnumParser.
+type Parser[K any] struct {
+	functions         map[string]Factory[K]
+	pathParser        PathExpressionParser[K]
+	enumParser        EnumParser
+	telemetrySettings component.TelemetrySettings
+}
+
 func NewParser[K any](
 	functions map[string]Factory[K],
 	pathParser PathExpressionParser[K],
@@ -143,7 +156,49 @@ func (p *Parser[K]) ParseStatement(statement string) (*Statement[K], error) {
 	}, nil
 }
 
+// ParseConditions parses string conditions into a Condition slice ready for execution.
+// Returns a slice of Condition and a nil error on successful parsing.
+// If parsing fails, returns nil and an error containing each error per failed condition.
+func (p *Parser[K]) ParseConditions(conditions []string) ([]*Condition[K], error) {
+	parsedConditions := make([]*Condition[K], 0, len(conditions))
+	var parseErrs []error
+
+	for _, condition := range conditions {
+		ps, err := p.ParseCondition(condition)
+		if err != nil {
+			parseErrs = append(parseErrs, fmt.Errorf("unable to parse OTTL condition %q: %w", condition, err))
+			continue
+		}
+		parsedConditions = append(parsedConditions, ps)
+	}
+
+	if len(parseErrs) > 0 {
+		return nil, errors.Join(parseErrs...)
+	}
+
+	return parsedConditions, nil
+}
+
+// ParseCondition parses a single string condition into a Condition objects ready for execution.
+// Returns an Condition and a nil error on successful parsing.
+// If parsing fails, returns nil and an error.
+func (p *Parser[K]) ParseCondition(condition string) (*Condition[K], error) {
+	parsed, err := parseCondition(condition)
+	if err != nil {
+		return nil, err
+	}
+	expression, err := p.newBoolExpr(parsed)
+	if err != nil {
+		return nil, err
+	}
+	return &Condition[K]{
+		condition: expression,
+		origText:  condition,
+	}, nil
+}
+
 var parser = newParser[parsedStatement]()
+var conditionParser = newParser[booleanExpression]()
 
 func parseStatement(raw string) (*parsedStatement, error) {
 	parsed, err := parser.ParseString("", raw)
@@ -159,6 +214,20 @@ func parseStatement(raw string) (*parsedStatement, error) {
 	return parsed, nil
 }
 
+func parseCondition(raw string) (*booleanExpression, error) {
+	parsed, err := conditionParser.ParseString("", raw)
+
+	if err != nil {
+		return nil, fmt.Errorf("condition has invalid syntax: %w", err)
+	}
+	err = parsed.checkForCustomError()
+	if err != nil {
+		return nil, err
+	}
+
+	return parsed, nil
+}
+
 // newParser returns a parser that can be used to read a string into a parsedStatement. An error will be returned if the string
 // is not formatted for the DSL.
 func newParser[G any]() *participle.Parser[G] {