Add documentation example for processor package (#5675) (#13070)

#### Description

This PR introduces a testable example to the package
[processor](/processor). it's a similar example to the documentation one
on
https://opentelemetry.io/docs/collector/building/receiver/#designing-and-validating-receiver-settings

#### Issue:
https://github.com/open-telemetry/opentelemetry-collector/issues/5675#issuecomment-2796741807

#### Testing

the testable example was executed successfully:


![image](https://github.com/user-attachments/assets/a57a2bc8-22f0-4063-a1b4-aac640423219)


#### Documentation

this testable example follows
https://go.dev/blog/examples#larger-examples format and should be
automatically added to `processor` package documentation

processor/README.md

@@ -11,6 +11,7 @@ Some important aspects of pipelines and processors to be aware of:
 - [Exclusive Ownership](#exclusive-ownership)
 - [Shared Ownership](#shared-ownership)
 - [Ordering Processors](#ordering-processors)
+- [Creating Custom Processor](#creating-custom-processors)
 
 Supported processors (sorted alphabetically):
 - [Batch Processor](batchprocessor/README.md)
@@ -107,3 +108,8 @@ data cloning described in Exclusive Ownership section.
 
 The order processors are specified in a pipeline is important as this is the
 order in which each processor is applied.
+
+## Creating Custom Processors
+
+To create a custom processor for the OpenTelemetry Collector, you need to implement the processor interface, define the processor's configuration, and register it with the Collector. The process typically involves creating a factory, implementing the required processing logic, and handling configuration options. For a practical example and guidance, refer to the [`processorhelper`](https://pkg.go.dev/go.opentelemetry.io/collector/processor/processorhelper) package, which provides utilities and patterns to simplify processor development.
+

processor/processorhelper/example_test.go

@@ -0,0 +1,99 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package processorhelper_test
+
+import (
+	"context"
+	"fmt"
+
+	"go.opentelemetry.io/collector/component"
+	"go.opentelemetry.io/collector/consumer"
+	"go.opentelemetry.io/collector/pdata/pmetric"
+	"go.opentelemetry.io/collector/processor"
+	"go.opentelemetry.io/collector/processor/processorhelper"
+)
+
+// typeStr defines the unique type identifier for the processor.
+var typeStr = component.MustNewType("example")
+
+// exampleConfig holds configuration settings for the processor.
+type exampleConfig struct{}
+
+// exampleProcessor implements the OpenTelemetry processor interface.
+type exampleProcessor struct {
+	cancel context.CancelFunc
+	config exampleConfig
+}
+
+// Example demonstrates the usage of the processor factory.
+func Example() {
+	// Instantiate the processor factory and print its type.
+	exampleProcessor := NewFactory()
+	fmt.Println(exampleProcessor.Type())
+
+	// Output:
+	// example
+}
+
+// NewFactory creates a new processor factory.
+func NewFactory() processor.Factory {
+	return processor.NewFactory(
+		typeStr,
+		createDefaultConfig,
+		processor.WithMetrics(createExampleProcessor, component.StabilityLevelAlpha),
+	)
+}
+
+// createDefaultConfig returns the default configuration for the processor.
+func createDefaultConfig() component.Config {
+	return &exampleConfig{}
+}
+
+// createExampleProcessor initializes an instance of the example processor.
+func createExampleProcessor(ctx context.Context, params processor.Settings, baseCfg component.Config, next consumer.Metrics) (processor.Metrics, error) {
+	// Convert baseCfg to the correct type.
+	cfg := baseCfg.(*exampleConfig)
+
+	// Create a new processor instance.
+	pcsr := newExampleProcessor(ctx, cfg)
+
+	// Wrap the processor with the helper utilities.
+	return processorhelper.NewMetrics(
+		ctx,
+		params,
+		cfg,
+		next,
+		pcsr.consumeMetrics,
+		processorhelper.WithCapabilities(consumer.Capabilities{MutatesData: true}),
+		processorhelper.WithShutdown(pcsr.shutdown),
+	)
+}
+
+// newExampleProcessor constructs a new instance of the example processor.
+func newExampleProcessor(ctx context.Context, cfg *exampleConfig) *exampleProcessor {
+	pcsr := &exampleProcessor{
+		config: *cfg,
+	}
+
+	// Create a cancelable context.
+	_, pcsr.cancel = context.WithCancel(ctx)
+
+	return pcsr
+}
+
+// ConsumeMetrics modify metrics adding one attribute to resource.
+func (pcsr *exampleProcessor) consumeMetrics(_ context.Context, md pmetric.Metrics) (pmetric.Metrics, error) {
+	rm := md.ResourceMetrics()
+	for i := 0; i < rm.Len(); i++ {
+		rm.At(i).Resource().Attributes().PutStr("processed_by", "exampleProcessor")
+	}
+
+	return md, nil
+}
+
+// Shutdown properly stops the processor and releases resources.
+func (pcsr *exampleProcessor) shutdown(_ context.Context) error {
+	pcsr.cancel()
+	return nil
+}