* initial metrics work
* rename cumulative to counter
* rename bidirectional to nonmonotonic
* rename unidirectional to monotonic
* rename nonnegative to signed
this changes the default semantics a bit - before the change measure
could record negative values by default, now it can't.
The specification draft currently specifies both NonNegative and
Signed, but I think it's a mistake.
* rename instrument to descriptor
* license
* rework measurement values
* make measurement value a tagged union
* simplify to one kind of metrics
* add observers
* change some interfaces to match the spec
* keep integral measurement separate from floating ones
* remove duplicated measurement type
* add checking for options
* reorder some fields and functions
* rename a function
to avoid confusion between the Handle type and the Measure type
* drop disabled field from descriptor
* add back typed API for metrics
* make metric options type safe
* merge alternatives into a single bool
* make value kind name less stuttery
* fix observation callback prototype
* drop context parameter from NewHandle
* drop useless parameter names
* make descriptor an opaque struct
* use a store helper
* handle comment fixes
* reword Alternate comment
* drop the "any value" metrics
* make measurement value simpler
* document value stuff
* add tests for values
* docs
* do not panic if there is no span ID in the event
* Move scope.Active to trace.CurrentSpan
* Remove scope / does not build
* Global tracer
* Checkpoint
* Checkpoint
* Add key/key.go for key.New
* Comments
* Remove more EventID and ScopeID
* Use Handle to describe static objects
* TODOs
* Remove empty file
* Remove singletons
* Update TODOs
* TODO about map update
* Make stats package option aliases (like key has)
* Rename experimental/streaming
* streaming SDK builds w/ many TODOs
* Get the examples building
* Tidy up metric API / add interface check
* Remove logic from the registry; this is now a placeholder
