Add example test for a custom SpanProcessor (#1196)

* Add example test for a custom SpanProcessor

* Use existing test framework exporter

* Update example

* Add fixes from upstream

sdk/trace/span_processor_example_test.go

@@ -0,0 +1,92 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package trace
+
+import (
+	"time"
+
+	export "go.opentelemetry.io/otel/sdk/export/trace"
+	"go.opentelemetry.io/otel/sdk/export/trace/tracetest"
+)
+
+// DurationFilter is a SpanProcessor that filters spans that have lifetimes
+// outside of a defined range.
+type DurationFilter struct {
+	// Next is the next SpanProcessor in the chain.
+	Next SpanProcessor
+
+	// Min is the duration under which spans are dropped.
+	Min time.Duration
+	// Max is the duration over which spans are dropped.
+	Max time.Duration
+}
+
+func (f DurationFilter) OnStart(sd *export.SpanData) { f.Next.OnStart(sd) }
+func (f DurationFilter) Shutdown()                   { f.Next.Shutdown() }
+func (f DurationFilter) ForceFlush()                 { f.Next.ForceFlush() }
+func (f DurationFilter) OnEnd(sd *export.SpanData) {
+	if f.Min > 0 && sd.EndTime.Sub(sd.StartTime) < f.Min {
+		// Drop short lived spans.
+		return
+	}
+	if f.Max > 0 && sd.EndTime.Sub(sd.StartTime) > f.Max {
+		// Drop long lived spans.
+		return
+	}
+	f.Next.OnEnd(sd)
+}
+
+// InstrumentationBlacklist is a SpanProcessor that drops all spans from
+// certain instrumentation.
+type InstrumentationBlacklist struct {
+	// Next is the next SpanProcessor in the chain.
+	Next SpanProcessor
+
+	// Blacklist is the set of instrumentation names for which spans will be
+	// dropped.
+	Blacklist map[string]bool
+}
+
+func (f InstrumentationBlacklist) OnStart(sd *export.SpanData) { f.Next.OnStart(sd) }
+func (f InstrumentationBlacklist) Shutdown()                   { f.Next.Shutdown() }
+func (f InstrumentationBlacklist) ForceFlush()                 { f.Next.ForceFlush() }
+func (f InstrumentationBlacklist) OnEnd(sd *export.SpanData) {
+	if f.Blacklist != nil && f.Blacklist[sd.InstrumentationLibrary.Name] {
+		// Drop spans from this instrumentation
+		return
+	}
+	f.Next.OnEnd(sd)
+}
+
+func ExampleSpanProcessor() {
+	exportSP := NewSimpleSpanProcessor(tracetest.NewNoopExporter())
+
+	// Build a SpanProcessor chain to filter out all spans from the pernicious
+	// "naughty-instrumentation" dependency and only allow spans shorter than
+	// an minute and longer than a second to be exported with the exportSP.
+	filter := DurationFilter{
+		Next: InstrumentationBlacklist{
+			Next: exportSP,
+			Blacklist: map[string]bool{
+				"naughty-instrumentation": true,
+			},
+		},
+		Min: time.Second,
+		Max: time.Minute,
+	}
+
+	_ = NewTracerProvider(WithSpanProcessor(filter))
+	// ...
+}