Add ottl len converter (#24420)

**Description:** 

Add ottl `Len` converter to make `Substring` useful for truncating.
E.g.:
```
    log_statements:
      - context: log
        statements:
        - set(body, Substring(body, 0, 10)) where Len(body) >= 10
```

**Link to tracking Issue:**  Resolves #23847.

**Testing:** Unit tests, local verification with running collector.

**Documentation:** Added documentation to
`pkg/ottl/ottlfuncs/README.md`.

This PR is the result of
[conversation](#23880 (comment))
on #23880.

---------

Co-authored-by: Tyler Helmuth <[email protected]>
Co-authored-by: Evan Bradley <[email protected]>

.chloggen/add-ottl-len-converter.yaml

@@ -0,0 +1,20 @@
+# Use this changelog template to create an entry for release notes.
+# If your change doesn't affect end users, such as a test fix or a tooling change,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+
+# 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 new `Len` converter that computes the length of strings, slices, and maps.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [23847]
+
+# (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:

pkg/ottl/ottlfuncs/README.md

@@ -459,6 +459,20 @@ Examples:
 
 - `IsString(attributes["maybe a string"])`
 
+### Len
+
+`Len(target)`
+
+The `Len` Converter returns the int64 length of the target string or slice.
+
+`target` is either a `string`, `slice`, `map`, `pcommon.Slice`, `pcommon.Map`, or `pcommon.Value` with type `pcommon.ValueTypeStr`, `pcommon.ValueTypeSlice`, or `pcommon.ValueTypeMap`.
+
+If the `target` is not an acceptable type, the `Len` Converter will return an error.
+
+Examples:
+
+- `Len(body)`
+
 ### Log
 
 `Log(value)`

pkg/ottl/ottlfuncs/func_len.go

@@ -0,0 +1,71 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package ottlfuncs // import "github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl/ottlfuncs"
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+
+	"go.opentelemetry.io/collector/pdata/pcommon"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+const (
+	typeError = "target arg must be of type string, []any, map[string]any, pcommon.Map, pcommon.Slice, or pcommon.Value (of type String, Map, Slice)"
+)
+
+type LenArguments[K any] struct {
+	Target ottl.Getter[K] `ottlarg:"0"`
+}
+
+func NewLenFactory[K any]() ottl.Factory[K] {
+	return ottl.NewFactory("Len", &LenArguments[K]{}, createLenFunction[K])
+}
+
+func createLenFunction[K any](_ ottl.FunctionContext, oArgs ottl.Arguments) (ottl.ExprFunc[K], error) {
+	args, ok := oArgs.(*LenArguments[K])
+
+	if !ok {
+		return nil, fmt.Errorf("LenFactory args must be of type *LenArguments[K]")
+	}
+
+	return computeLen(args.Target), nil
+}
+
+// nolint:exhaustive
+func computeLen[K any](target ottl.Getter[K]) ottl.ExprFunc[K] {
+	return func(ctx context.Context, tCtx K) (interface{}, error) {
+		val, err := target.Get(ctx, tCtx)
+		if err != nil {
+			return nil, err
+		}
+
+		switch valType := val.(type) {
+		case pcommon.Value:
+			switch valType.Type() {
+			case pcommon.ValueTypeStr:
+				return int64(len(valType.Str())), nil
+			case pcommon.ValueTypeSlice:
+				return int64(valType.Slice().Len()), nil
+			case pcommon.ValueTypeMap:
+				return int64(valType.Map().Len()), nil
+			}
+			return nil, fmt.Errorf(typeError)
+		case pcommon.Map:
+			return int64(valType.Len()), nil
+		case pcommon.Slice:
+			return int64(valType.Len()), nil
+		}
+
+		v := reflect.ValueOf(val)
+		switch v.Kind() {
+		case reflect.String, reflect.Map, reflect.Slice:
+			return int64(v.Len()), nil
+		}
+
+		return nil, fmt.Errorf(typeError)
+	}
+}

pkg/ottl/ottlfuncs/func_len_test.go

@@ -0,0 +1,127 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package ottlfuncs
+
+import (
+	"context"
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.opentelemetry.io/collector/pdata/pcommon"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+func Test_Len(t *testing.T) {
+	pcommonSlice := pcommon.NewSlice()
+	err := pcommonSlice.FromRaw(make([]any, 5))
+	if err != nil {
+		t.Error(err)
+	}
+
+	pcommonMap := pcommon.NewMap()
+	err = pcommonMap.FromRaw(dummyMap(5))
+	if err != nil {
+		t.Error(err)
+	}
+
+	pcommonValueSlice := pcommon.NewValueSlice()
+	err = pcommonValueSlice.FromRaw(make([]any, 5))
+	if err != nil {
+		t.Error(err)
+	}
+
+	pcommonValueMap := pcommon.NewValueMap()
+	err = pcommonValueMap.FromRaw(dummyMap(5))
+	if err != nil {
+		t.Error(err)
+	}
+
+	tests := []struct {
+		name     string
+		value    interface{}
+		expected int64
+	}{
+		{
+			name:     "string",
+			value:    "a string",
+			expected: 8,
+		},
+		{
+			name:     "map",
+			value:    dummyMap(5),
+			expected: 5,
+		},
+		{
+			name:     "string slice",
+			value:    make([]string, 5),
+			expected: 5,
+		},
+		{
+			name:     "int slice",
+			value:    make([]int, 5),
+			expected: 5,
+		},
+		{
+			name:     "pcommon map",
+			value:    pcommonMap,
+			expected: 5,
+		},
+		{
+			name:     "pcommon slice",
+			value:    pcommonSlice,
+			expected: 5,
+		},
+		{
+			name:     "pcommon value string",
+			value:    pcommon.NewValueStr("a string"),
+			expected: 8,
+		},
+		{
+			name:     "pcommon value slice",
+			value:    pcommonValueSlice,
+			expected: 5,
+		},
+		{
+			name:     "pcommon value map",
+			value:    pcommonValueMap,
+			expected: 5,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			exprFunc := computeLen[any](&ottl.StandardGetSetter[any]{
+				Getter: func(context context.Context, tCtx any) (interface{}, error) {
+					return tt.value, nil
+				},
+			})
+			result, err := exprFunc(context.Background(), nil)
+			assert.NoError(t, err)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func dummyMap(size int) map[string]any {
+	m := make(map[string]any, size)
+	for i := 0; i < size; i++ {
+		m[strconv.Itoa(i)] = i
+	}
+	return m
+}
+
+// nolint:errorlint
+func Test_Len_Error(t *testing.T) {
+	exprFunc := computeLen[any](&ottl.StandardGetSetter[any]{
+		Getter: func(context.Context, interface{}) (interface{}, error) {
+			return 24, nil
+		},
+	})
+	result, err := exprFunc(context.Background(), nil)
+	assert.Nil(t, result)
+	assert.Error(t, err)
+	_, ok := err.(ottl.TypeError)
+	assert.False(t, ok)
+}

pkg/ottl/ottlfuncs/functions.go

@@ -42,6 +42,7 @@ func converters[K any]() []ottl.Factory[K] {
 		NewIsMapFactory[K](),
 		NewIsMatchFactory[K](),
 		NewIsStringFactory[K](),
+		NewLenFactory[K](),
 		NewLogFactory[K](),
 		NewParseJSONFactory[K](),
 		NewSHA1Factory[K](),