Add AlwaysParentSample sampler (#455)

* Add AlwaysParentSample

* Update sampler doc

Co-Authored-By: Krzesimir Nowak <qdlacz@gmail.com>

* Address PR comments

Co-authored-by: Krzesimir Nowak <qdlacz@gmail.com>
Co-authored-by: Rahul Patel <rghetia@yahoo.com>

sdk/trace/sampling.go

@@ -77,3 +77,11 @@ func NeverSample() Sampler {
 		return SamplingDecision{Sample: false}
 	}
 }
+
+// AlwaysParentSample returns a Sampler that samples a trace only
+// if the parent span is sampled.
+// This Sampler is a passthrough to the ProbabilitySampler with
+// a fraction of value 0.
+func AlwaysParentSample() Sampler {
+	return ProbabilitySampler(0)
+}

sdk/trace/sampling_test.go

@@ -0,0 +1,49 @@
+// Copyright 2020, 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_test
+
+import (
+	"testing"
+
+	"go.opentelemetry.io/otel/api/core"
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+func TestAlwaysParentSampleWithParentSampled(t *testing.T) {
+	sampler := sdktrace.AlwaysParentSample()
+	traceID, _ := core.TraceIDFromHex("4bf92f3577b34da6a3ce929d0e0e4736")
+	spanID, _ := core.SpanIDFromHex("00f067aa0ba902b7")
+	parentCtx := core.SpanContext{
+		TraceID:    traceID,
+		SpanID:     spanID,
+		TraceFlags: core.TraceFlagsSampled,
+	}
+	if !sampler(sdktrace.SamplingParameters{ParentContext: parentCtx}).Sample {
+		t.Error("Sampling decision should be true")
+	}
+}
+
+func TestAlwaysParentSampleWithParentNotSampled(t *testing.T) {
+	sampler := sdktrace.AlwaysParentSample()
+	traceID, _ := core.TraceIDFromHex("4bf92f3577b34da6a3ce929d0e0e4736")
+	spanID, _ := core.SpanIDFromHex("00f067aa0ba902b7")
+	parentCtx := core.SpanContext{
+		TraceID: traceID,
+		SpanID:  spanID,
+	}
+	if sampler(sdktrace.SamplingParameters{ParentContext: parentCtx}).Sample {
+		t.Error("Sampling decision should be false")
+	}
+}