Combined Metrics API updates for v0.3 (Add Observer instrument, remov…

…e Gauge instrument, etc.) (open-telemetry#430)

* Gauge/Observer rewrite pt 1

* Checkpoint

* Checkpoint

* Optional features restored

* Update selection process

* Update TOC

* Reword intro para on standard impl

* Add detail on label set

* Add examples

* Misspellings

* Rewrite the introductory material

* Add a section on time

* Clarify 'standard implementation' and 'default interpretation'

* Explain aggregations; Discourage timestamp use

* Reword the metric event format

* Update Counter and Measure

* Update Observer (partial)

* Remove options, discuss views API

* Fixes

* Fixes

* Fixes

* Rename BoundTimer

* Typos and suggestions

* Reword global Meter

* Rearrange early paragraphs

* Use _run time_ and _simultaneous_

* Rearrange early paragraphs (again)

* Define _run time_

* These

* Iterate

* Remove a TODO

* More on global/named meters

* Difference, Capture

* Timer wording

* Explicit timestamp

* Rewrite simultaneous

* Example: active requests

* Synchronously

* Wording

* Example using correlation context

* Detail on WithKeys

* Counter is a special case

* Counter fixes; typo

* Typo

* Split the leading example into Counter and Measure cases

* Remove poor example

* Update TOC

* Add note about Observer and Context

* Add note about context-freedom in Observer

* Run MDL

* WithRecommendedKeys

* Typo

* Address most of Bogdan's feedback

* Typo

Co-authored-by: Bogdan Drutu <lazy@splunk.com>