Trace sdk (#65)

* trace sdk initial commit.

* fix imports and comments.

* remove tracestate

* split trace.go

* add attribute over limit test.

* add comments and restructure span.go and tracer.go

* refactor MessageEvent

* defer unlock

* some more cleanup in span.go

* rename *MessageEvent* to *Event*

* cleanup comments in trace_test.go

* fix typos.

* return full string ID for traceID and spanID.

sdk/trace/sampling.go

@@ -0,0 +1,75 @@
+// 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 defaultSamplingProbability = 1e-4
+
+// Sampler decides whether a trace should be sampled and exported.
+type Sampler func(SamplingParameters) SamplingDecision
+
+// SamplingParameters contains the values passed to a Sampler.
+type SamplingParameters struct {
+	ParentContext   core.SpanContext
+	TraceID         core.TraceID
+	SpanID          uint64
+	Name            string
+	HasRemoteParent bool
+}
+
+// SamplingDecision is the value returned by a Sampler.
+type SamplingDecision struct {
+	Sample bool
+}
+
+// ProbabilitySampler returns a Sampler that samples a given fraction of traces.
+//
+// It also samples spans whose parents are sampled.
+func ProbabilitySampler(fraction float64) Sampler {
+	if !(fraction >= 0) {
+		fraction = 0
+	} else if fraction >= 1 {
+		return AlwaysSample()
+	}
+
+	traceIDUpperBound := uint64(fraction * (1 << 63))
+	return Sampler(func(p SamplingParameters) SamplingDecision {
+		if p.ParentContext.IsSampled() {
+			return SamplingDecision{Sample: true}
+		}
+		x := p.TraceID.High >> 1
+		return SamplingDecision{Sample: x < traceIDUpperBound}
+	})
+}
+
+// AlwaysSample returns a Sampler that samples every trace.
+// Be careful about using this sampler in a production application with
+// significant traffic: a new trace will be started and exported for every
+// request.
+func AlwaysSample() Sampler {
+	return func(p SamplingParameters) SamplingDecision {
+		return SamplingDecision{Sample: true}
+	}
+}
+
+// NeverSample returns a Sampler that samples no traces.
+func NeverSample() Sampler {
+	return func(p SamplingParameters) SamplingDecision {
+		return SamplingDecision{Sample: false}
+	}
+}