sdk/log: Add filtering Processor example (#5543)

Towards https://github.com/open-telemetry/opentelemetry-go/issues/5065

sdk/log/example_test.go

@@ -11,6 +11,55 @@ import (
 	logsdk "go.opentelemetry.io/otel/sdk/log"
 )
 
+// Use a processor that filters out records based on the provided context.
+func ExampleProcessor_filtering() {
+	// Existing processor that emits telemetry.
+	var processor logsdk.Processor = logsdk.NewBatchProcessor(nil)
+
+	// Wrap the processor so that it ignores processing log records
+	// when a context deriving from WithIgnoreLogs is passed
+	// to the logging methods.
+	processor = &ContextFilterProcessor{processor}
+
+	// The created processor can then be registered with
+	// the OpenTelemetry Logs SDK using the WithProcessor option.
+	_ = logsdk.NewLoggerProvider(
+		logsdk.WithProcessor(processor),
+	)
+}
+
+type key struct{}
+
+var igoreLogsKey key
+
+// WithIgnoreLogs returns a context which is used by [ContextFilterProcessor]
+// to filter out log records.
+func WithIgnoreLogs(ctx context.Context) context.Context {
+	return context.WithValue(ctx, igoreLogsKey, true)
+}
+
+// ContextFilterProcessor filters out logs when a context deriving from
+// [WithIgnoreLogs] is passed to its methods.
+type ContextFilterProcessor struct {
+	logsdk.Processor
+}
+
+func (p *ContextFilterProcessor) OnEmit(ctx context.Context, record logsdk.Record) error {
+	if ignoreLogs(ctx) {
+		return nil
+	}
+	return p.Processor.OnEmit(ctx, record)
+}
+
+func (p *ContextFilterProcessor) Enabled(ctx context.Context, record logsdk.Record) bool {
+	return !ignoreLogs(ctx) && p.Processor.Enabled(ctx, record)
+}
+
+func ignoreLogs(ctx context.Context) bool {
+	_, ok := ctx.Value(igoreLogsKey).(bool)
+	return ok
+}
+
 // Use a processor which redacts sensitive data from some attributes.
 func ExampleProcessor_redact() {
 	// Existing processor that emits telemetry.