[pkg/ottl] Add Base64Decode function (open-telemetry#31543)

**Description:** Adds a new `Base64Decode` function to facilitate ingest of base64 encoded data

**Link to tracking Issue:** open-telemetry#31543

**Testing:**  Added unit test

**Documentation:**  Updated the func readme.

Author: DougManton

.chloggen/31543-add-base64decode-function-ottl.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: ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Add new function to decode a base64 encoded string and output the original string"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [31543]
+
+# (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: []

pkg/ottl/ottlfuncs/README.md

@@ -377,6 +377,7 @@ Unlike functions, they do not modify any input telemetry and always return a val
 
 Available Converters:
 
+- [Base64Decode](#base64decode)
 - [Concat](#concat)
 - [ConvertCase](#convertcase)
 - [ExtractPatterns](#extractpatterns)
@@ -417,6 +418,21 @@ Available Converters:
 - [UnixSeconds](#unixseconds)
 - [UUID](#UUID)
 
+### Base64Decode
+
+`Base64Decode(value)`
+
+The `Base64Decode` Converter takes a base64 encoded string and returns the decoded string.
+
+`value` is a valid base64 encoded string.
+
+Examples:
+
+- `Base64Decode("aGVsbG8gd29ybGQ=")`
+
+
+- `Base64Decode(attributes["encoded field"])`
+
 ### Concat
 
 `Concat(values[], delimiter)`

pkg/ottl/ottlfuncs/func_base64decode.go

@@ -0,0 +1,45 @@
+// 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"
+	"encoding/base64"
+	"fmt"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+type Base64DecodeArguments[K any] struct {
+	Target ottl.StringGetter[K]
+}
+
+func NewBase64DecodeFactory[K any]() ottl.Factory[K] {
+	return ottl.NewFactory("Base64Decode", &Base64DecodeArguments[K]{}, createBase64DecodeFunction[K])
+}
+
+func createBase64DecodeFunction[K any](_ ottl.FunctionContext, oArgs ottl.Arguments) (ottl.ExprFunc[K], error) {
+	args, ok := oArgs.(*Base64DecodeArguments[K])
+
+	if !ok {
+		return nil, fmt.Errorf("Base64DecodeFactory args must be of type *Base64DecodeArguments[K]")
+	}
+
+	return Base64Decode(args.Target)
+}
+
+func Base64Decode[K any](target ottl.StringGetter[K]) (ottl.ExprFunc[K], error) {
+
+	return func(ctx context.Context, tCtx K) (any, error) {
+		val, err := target.Get(ctx, tCtx)
+		if err != nil {
+			return nil, err
+		}
+		base64string, err := base64.StdEncoding.DecodeString(val)
+		if err != nil {
+			return nil, err
+		}
+		return string(base64string), nil
+	}, nil
+}

pkg/ottl/ottlfuncs/func_base64decode_test.go

@@ -0,0 +1,82 @@
+// 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_Base64Decode(t *testing.T) {
+	tests := []struct {
+		name     string
+		value    any
+		expected any
+		err      bool
+	}{
+		{
+			name:     "base64-string",
+			value:    "aGVsbG8gd29ybGQ=",
+			expected: "hello world",
+		},
+		{
+			name:     "empty string",
+			value:    "",
+			expected: "",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			exprFunc, err := Base64Decode[any](&ottl.StandardStringGetter[any]{
+				Getter: func(context.Context, any) (any, error) {
+					return tt.value, nil
+				},
+			})
+			assert.NoError(t, err)
+			result, err := exprFunc(nil, nil)
+			if tt.err {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func Test_Base64DecodeError(t *testing.T) {
+	tests := []struct {
+		name          string
+		value         any
+		err           bool
+		expectedError string
+	}{
+		{
+			name:          "non-string",
+			value:         10,
+			expectedError: "expected string but got int",
+		},
+		{
+			name:          "nil",
+			value:         nil,
+			expectedError: "expected string but got nil",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			exprFunc, err := Base64Decode[any](&ottl.StandardStringGetter[any]{
+				Getter: func(context.Context, any) (any, error) {
+					return tt.value, nil
+				},
+			})
+			assert.NoError(t, err)
+			_, err = exprFunc(nil, nil)
+			assert.ErrorContains(t, err, tt.expectedError)
+		})
+	}
+}

pkg/ottl/ottlfuncs/functions.go

@@ -35,6 +35,7 @@ func StandardConverters[K any]() map[string]ottl.Factory[K] {
 func converters[K any]() []ottl.Factory[K] {
 	return []ottl.Factory[K]{
 		// Converters
+		NewBase64DecodeFactory[K](),
 		NewConcatFactory[K](),
 		NewConvertCaseFactory[K](),
 		NewDoubleFactory[K](),