Dogstatsd metrics exporter (#326)

* Add MetricAggregator.Merge() implementations

* Update from feedback

* Type

* Ckpt

* Ckpt

* Add push controller

* Ckpt

* Add aggregator interfaces, stdout encoder

* Modify basic main.go

* Main is working

* Batch stdout output

* Sum udpate

* Rename stdout

* Add stateless/stateful Batcher options

* Undo a for-loop in the example, remove a done TODO

* Update imports

* Add note

* Rename defaultkeys

* Support variable label encoder to speed OpenMetrics/Statsd export

* Lint

* Checkpoint

* Checkpoint

* Doc

* Precommit/lint

* Simplify Aggregator API

* Record->Identifier

* Remove export.Record a.k.a. Identifier

* Checkpoint

* Propagate errors to the SDK, remove a bunch of 'TODO warn'

* Checkpoint

* Introduce export.Labels

* Comments in export/metric.go

* Comment

* More merge

* More doc

* Complete example

* Lint fixes

* Add a testable example

* Lint

* Dogstats

* Let Export return an error

* Checkpoint

* add a basic stdout exporter test

* Add measure test; fix aggregator APIs

* Use JSON numbers, not strings

* Test stdout exporter error

* Add a test for the call to RangeTest

* Add error handler API to improve correctness test; return errors from RecordOne

* Undo the previous -- do not expose errors

* Add simple selector variations, test

* Repair examples

* Test push controller error handling

* Add SDK label encoder tests

* Add a defaultkeys batcher test

* Add an ungrouped batcher test

* Lint new tests

* Respond to krnowak's feedback

* Checkpoint

* Funciontal example using unixgram

* Tidy the example

* Add a packet-split test

* More tests

* Undo comment

* Use concrete receivers for export records and labels, since the constructors return structs not pointers

* Bug fix for stateful batchers; clone an aggregator for long term storage

* Remove TODO addressed in #318

* Add errors to all aggregator interfaces

* Handle ErrNoLastValue case in stdout exporter

* Move aggregator API into sdk/export/metric/aggregator

* Update all aggregator exported-method comments

* Document the aggregator APIs

* More aggregator comments

* Add multiple updates to the ungrouped test

* Fixes for feedback from Gustavo and Liz

* Producer->CheckpointSet; add FinishedCollection

* Process takes an export.Record

* ReadCheckpoint->CheckpointSet

* EncodeLabels->Encode

* Format a better inconsistent type error; add more aggregator API tests

* More RangeTest test coverage

* Make benbjohnson/clock a test-only dependency

* Handle ErrNoLastValue in stress_test

* Update comments; use a pipe vs a unix socket in the example test

* Update test

* Spelling

* Typo fix

* Rename DefaultLabelEncoder to NewDefaultLabelEncoder for clarity

* Rename DefaultLabelEncoder to NewDefaultLabelEncoder for clarity

* Test different adapters; add ForceEncode to statsd label encoder

sdk/metric/aggregator/array/array.go

@@ -29,17 +29,18 @@ import (
 type (
 	Aggregator struct {
 		lock       sync.Mutex
-		current    Points
-		checkpoint Points
+		current    points
+		checkpoint points
 		ckptSum    core.Number
 	}
 
-	Points []core.Number
+	points []core.Number
 )
 
 var _ export.Aggregator = &Aggregator{}
 var _ aggregator.MaxSumCount = &Aggregator{}
 var _ aggregator.Distribution = &Aggregator{}
+var _ aggregator.Points = &Aggregator{}
 
 // New returns a new array aggregator, which aggregates recorded
 // measurements by storing them in an array.  This type uses a mutex
@@ -74,6 +75,11 @@ func (c *Aggregator) Quantile(q float64) (core.Number, error) {
 	return c.checkpoint.Quantile(q)
 }
 
+// Points returns access to the raw data set.
+func (c *Aggregator) Points() ([]core.Number, error) {
+	return c.checkpoint, nil
+}
+
 // Checkpoint saves the current state and resets the current state to
 // the empty set, taking a lock to prevent concurrent Update() calls.
 func (c *Aggregator) Checkpoint(ctx context.Context, desc *export.Descriptor) {
@@ -133,8 +139,8 @@ func (c *Aggregator) sort(kind core.NumberKind) {
 	}
 }
 
-func combine(a, b Points, kind core.NumberKind) Points {
-	result := make(Points, 0, len(a)+len(b))
+func combine(a, b points, kind core.NumberKind) points {
+	result := make(points, 0, len(a)+len(b))
 
 	for len(a) != 0 && len(b) != 0 {
 		if a[0].CompareNumber(kind, b[0]) < 0 {
@@ -150,25 +156,25 @@ func combine(a, b Points, kind core.NumberKind) Points {
 	return result
 }
 
-func (p *Points) Len() int {
+func (p *points) Len() int {
 	return len(*p)
 }
 
-func (p *Points) Less(i, j int) bool {
+func (p *points) Less(i, j int) bool {
 	// Note this is specialized for int64, because float64 is
 	// handled by `sort.Float64s` and uint64 numbers never appear
 	// in this data.
 	return int64((*p)[i]) < int64((*p)[j])
 }
 
-func (p *Points) Swap(i, j int) {
+func (p *points) Swap(i, j int) {
 	(*p)[i], (*p)[j] = (*p)[j], (*p)[i]
 }
 
 // Quantile returns the least X such that Pr(x<X)>=q, where X is an
 // element of the data set.  This uses the "Nearest-Rank" definition
 // of a quantile.
-func (p *Points) Quantile(q float64) (core.Number, error) {
+func (p *points) Quantile(q float64) (core.Number, error) {
 	if len(*p) == 0 {
 		return core.Number(0), aggregator.ErrEmptyDataSet
 	}