* 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
* Remove globals from exp/streaming
* basic example
* Build the streaming example
* Update README.md for running streaming example
* Remove observer package
* Move observer to exporter
* Fix
* Re-run make circle-ci
