Add Sampler API (#70)

- also add implementation of AlwaysSampleSampler and NeverSampleSampler

api/trace/never_sampler_test.go

@@ -0,0 +1,39 @@
+// Copyright 2019, 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 (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"go.opentelemetry.io/api/core"
+)
+
+func TestNeverSamperShouldSample(t *testing.T) {
+	gotD := NeverSampleSampler().ShouldSample(
+		core.SpanContext{}, false, core.TraceID{}, 0, "span")
+	wantD := Decision{Sampled: false}
+	if diff := cmp.Diff(wantD, gotD); diff != "" {
+		t.Errorf("Decision: +got, -want%v", diff)
+	}
+}
+
+func TestNeverSamplerDescription(t *testing.T) {
+	gotDesc := NeverSampleSampler().Description()
+	wantDesc := neverSamplerDescription
+	if diff := cmp.Diff(wantDesc, gotDesc); diff != "" {
+		t.Errorf("Description: +got, -want%v", diff)
+	}
+}