* Adds metric prototype

Fixes open-telemetry#1835

* Fix docs

* Add API metrics doc

* Add missing docs

* Add files

* Adding docs

* Refactor to _initialize

* Refactor initialize

* Add more documentation

* Add exporter test

* Add process

* Fix tests

* Try to add aggregator_class argument

Tests are failing here

* Fix instrument parent classes

* Test default aggregator

* WIP

* Add prototype test

* Tests passing again

* Use right counters

* All tests passing

* Rearrange instrument storage

* Fix tests

* Add HTTP server test

* WIP

* WIP

* Add prototype

* WIP

* Fail the test

* WIP

* WIP

* WIP

* WIP

* Add views

* Discard instruments via views

* Fix tests

* WIP

* WIP

* Fix lint

* WIP

* Fix test

* Fix lint

* Fix method

* Fix lint

* Mypy workaround

* Skip if 3.6

* Fix lint

* Add reason

* Fix 3.6

* Fix run

* Fix lint

* Remove SDK metrics

* Remove SDK docs

* Remove metrics

* Remove assertnotraises mixin

* Revert sdk docs conf

* Remove SDK env var changes

* Fix unit checking

* Define positional-only arguments

* Add Metrics plans

* Add API tests

* WIP

* WIP test

* WIP

* WIP

* WIP

* Set provider test passing

* Use a fixture

* Add test for get_provider

* Rename tests

* WIP

* WIP

* WIP

* WIP

* Remove non specific requirement

* Add meter requirements

* Put all meter provider tests in one file

* Add meter tests

* Make attributes be passed as a dictionary

* Make some interfaces private

* Log an error instead

* Remove ASCII flag

* Add CHANGELOG entry

* Add instrument tests

* All tests passing

* Add test

* Add name tests

* Add unit tests

* Add description tests

* Add counter tests

* Add more tests

* Add Histogram tests

* Add observable gauge tests

* Add updowncounter tests

* Add observableupdowncounter tests

* Fix lint

* Fix docs

* Fix lint

* Ignore mypy

* Remove useless pylint skip

* Remove useless pylint skip

* Remove useless pylint skip

* Remove useless pylint skip

* Remove useless pylint skip

* Add locks to meter and meterprovider

* Add lock to instruments

* Fix fixmes

* Fix lint

* Add documentation placeholder

* Remove blank line as requested.

* Do not override Rlock

* Remove unecessary super calls

* Add missing super calls

* Remove plan files

* Add missing parameters

* Rename observe to callback

* Fix lint

* Rename to secure_instrument_name

* Remove locks

* Fix lint

* Remove args and kwargs

* Remove implementation that gives meters access to meter provider

* Allow creating async instruments with either a callback function or generator

* add additional test with callback form of observable counter

* add a test/example that reads measurements from proc stat

* implement cpu time integration test with generator too

Co-authored-by: Aaron Abbott <aaronabbott@google.com>

* Make Measurement a concrete class

* comments

* update changelog

Co-authored-by: Aaron Abbott <aaronabbott@google.com>