* 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
* 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
* Let Export return an error
* 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
* 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
