[pkg/ottl] add string_converter (#31790)

**Description:** 
- implemented func_string (string_converter)
- added new unit tests in `pkg/ottl/ottlfuncs/func_string_test.go`
- added function to `pkg/ottl/ottlfuncs/functions.go`
- updatet README.md at `pkg/ottl/ottlfuncs/README.md`
- added changelog

**Link to tracking Issue:** 
- #27867 

**Testing:** 

- added similar tests as in `pkg/ottl/ottlfuncs/func_double_test.go`


**Documentation:** <Describe the documentation added.>

---------

Co-authored-by: Tyler Helmuth <12352919+TylerHelmuth@users.noreply.github.com>

.chloggen/string-converter.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: "Adding a string converter into pkg/ottl"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [27867]
+
+# (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: [user]

pkg/ottl/e2e/e2e_test.go

@@ -534,6 +534,36 @@ func Test_e2e_converters(t *testing.T) {
 				s.AppendEmpty().SetStr("C")
 			},
 		},
+		{
+			statement: `set(attributes["test"], String("test"))`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "test")
+			},
+		},
+		{
+			statement: `set(attributes["test"], String(attributes["http.method"]))`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "get")
+			},
+		},
+		{
+			statement: `set(attributes["test"], String(span_id))`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "[1,2,3,4,5,6,7,8]")
+			},
+		},
+		{
+			statement: `set(attributes["test"], String([1,2,3]))`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "[1,2,3]")
+			},
+		},
+		{
+			statement: `set(attributes["test"], String(true))`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "true")
+			},
+		},
 		{
 			statement: `set(attributes["test"], Substring("pass", 0, 2))`,
 			want: func(tCtx ottllog.TransformContext) {

pkg/ottl/ottlfuncs/README.md

@@ -409,6 +409,7 @@ Available Converters:
 - [SHA256](#sha256)
 - [SpanID](#spanid)
 - [Split](#split)
+- [String](#string)
 - [Substring](#substring)
 - [Time](#time)
 - [TraceID](#traceid)
@@ -1069,6 +1070,33 @@ Examples:
 
 - `Split("A|B|C", "|")`
 
+### String
+
+`String(value)`
+
+The `String` Converter converts the `value` to string type.
+
+The returned type is `string`.
+
+- string. The function returns the `value` without changes.
+- []byte. The function returns the `value` as a string encoded in hexadecimal.
+- map. The function returns the `value` as a key-value-pair of type string.
+- slice. The function returns the `value` as a list formatted string.
+- pcommon.Value. The function returns the `value` as a string type.
+
+If `value` is of another type it gets marshalled to string type.
+If `value` is empty, or parsing failed, nil is always returned.
+
+The `value` is either a path expression to a telemetry field to retrieve, or a literal.
+
+Examples:
+
+- `String("test")`
+- `String(attributes["http.method"])`
+- `String(span_id)`
+- `String([1,2,3])`
+- `String(false)`
+
 ### Substring
 
 `Substring(target, start, length)`

pkg/ottl/ottlfuncs/func_string.go

@@ -0,0 +1,42 @@
+// 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"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+type StringArguments[K any] struct {
+	Target ottl.StringLikeGetter[K]
+}
+
+func NewStringFactory[K any]() ottl.Factory[K] {
+	return ottl.NewFactory("String", &StringArguments[K]{}, createStringFunction[K])
+}
+
+func createStringFunction[K any](_ ottl.FunctionContext, oArgs ottl.Arguments) (ottl.ExprFunc[K], error) {
+	args, ok := oArgs.(*StringArguments[K])
+
+	if !ok {
+		return nil, fmt.Errorf("StringFactory args must be of type *StringArguments[K]")
+	}
+
+	return stringFunc(args.Target), nil
+}
+
+func stringFunc[K any](target ottl.StringLikeGetter[K]) ottl.ExprFunc[K] {
+	return func(ctx context.Context, tCtx K) (any, error) {
+		value, err := target.Get(ctx, tCtx)
+		if err != nil {
+			return nil, err
+		}
+		if value == nil {
+			return nil, nil
+		}
+		return *value, nil
+	}
+}

pkg/ottl/ottlfuncs/func_string_test.go

@@ -0,0 +1,99 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package ottlfuncs
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+func Test_String(t *testing.T) {
+	tests := []struct {
+		name     string
+		value    any
+		expected any
+		err      bool
+	}{
+		{
+			name:     "string",
+			value:    "test",
+			expected: string("test"),
+		},
+		{
+			name:     "empty string",
+			value:    "",
+			expected: string(""),
+		},
+		{
+			name:     "a number string",
+			value:    "333",
+			expected: string("333"),
+		},
+		{
+			name:     "int64",
+			value:    int64(333),
+			expected: string("333"),
+		},
+		{
+			name:     "float64",
+			value:    float64(2.7),
+			expected: string("2.7"),
+		},
+		{
+			name:     "float64 without decimal",
+			value:    float64(55),
+			expected: string("55"),
+		},
+		{
+			name:     "true",
+			value:    true,
+			expected: string("true"),
+		},
+		{
+			name:     "false",
+			value:    false,
+			expected: string("false"),
+		},
+		{
+			name:     "nil",
+			value:    nil,
+			expected: nil,
+		},
+		{
+			name:     "byte",
+			value:    []byte{123},
+			expected: string("7b"),
+		},
+		{
+			name:     "map",
+			value:    map[int]bool{1: true, 2: false},
+			expected: string("{\"1\":true,\"2\":false}"),
+		},
+		{
+			name:     "slice",
+			value:    []int{1, 2, 3},
+			expected: string("[1,2,3]"),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			exprFunc := stringFunc(&ottl.StandardStringLikeGetter[any]{
+				Getter: func(context.Context, any) (any, error) {
+					return tt.value, nil
+				},
+			})
+			result, err := exprFunc(nil, nil)
+			if tt.err {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}

pkg/ottl/ottlfuncs/functions.go

@@ -67,6 +67,7 @@ func converters[K any]() []ottl.Factory[K] {
 		NewSHA256Factory[K](),
 		NewSpanIDFactory[K](),
 		NewSplitFactory[K](),
+		NewStringFactory[K](),
 		NewSubstringFactory[K](),
 		NewTimeFactory[K](),
 		NewTruncateTimeFactory[K](),