Add Sampler API (#70)

- also add implementation of AlwaysSampleSampler and NeverSampleSampler

api/trace/always_sampler.go

@@ -0,0 +1,52 @@
+// 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 (
+	"go.opentelemetry.io/api/core"
+)
+
+const (
+	alwaysSamplerDescription = "AlwaysSampleSampler"
+)
+
+var alwaysSampleDecision = Decision{Sampled: true}
+
+type alwaysSampleSampler struct{}
+
+// ShouldSample implements Sampler interface.
+// It always returns a Decision with Sampled value set to true
+// and with Attributes set to an empty slice.
+func (as alwaysSampleSampler) ShouldSample(
+	_ core.SpanContext,
+	_ bool,
+	_ core.TraceID,
+	_ uint64,
+	_ string,
+) Decision {
+	return alwaysSampleDecision
+}
+
+// Description implements Sampler interface.
+// It returns the description of this sampler.
+func (as alwaysSampleSampler) Description() string {
+	return alwaysSamplerDescription
+}
+
+var _ Sampler = alwaysSampleSampler{}
+
+func AlwaysSampleSampler() alwaysSampleSampler {
+	return alwaysSampleSampler{}
+}