Add Sampler API (#70)

- also add implementation of AlwaysSampleSampler and NeverSampleSampler

api/trace/sampler.go

@@ -0,0 +1,44 @@
+// 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"
+
+type Sampler interface {
+	// ShouldSample returns a Decision that contains a decision whether to sample
+	// or not sample the span to be created. Decision is based on a Sampler specific
+	// algorithm that takes into account one or more input parameters.
+	ShouldSample(
+		sc core.SpanContext,
+		remote bool,
+		traceID core.TraceID,
+		spanID uint64,
+		spanName string,
+	) Decision
+
+	// Description returns of the sampler. It contains its name or short description
+	// and its configured properties.
+	// For example 'ProbabilitySampler:{0.00001}'
+	Description() string
+}
+
+type Decision struct {
+	// Sampled is set true if the span should be sampled.
+	Sampled bool
+
+	// Attributes provides insight into Sample	r's decision process.
+	// It could be empty slice or nil if no attributes are recorded by the sampler.
+	Attributes []core.KeyValue
+}