Metric API v0.4 specification (#581)

* Rewrite the intro

* Edit labels section

* Aggregations update

* Time update

* Three categories

* 6 kinds

* Section on memory requirements

* Update links to in-progress work

* Update TOC

* Reorder and update TOC

* Restore some text

* Restore the TODO code

* Restore api-user.md; for a separate PR

* Feedback round 1

* New pargraph

* Lint

* Remove six

* Remove the other two documents as out-of-date

* Update links; TODOs about api-user.md content lost

* Be consistent about distributed context and request

* Define Additive

* Remove refs to api-user.md

* Clarify that monotonics are additive

* Monotonic comment about rates

* Feedback

* SPI note

* Typos

* on correlation values

* remove content on memory requirements, it's SDK stuff

* Remove a dead link

* Update TOC

* Lint

Author: jmacd

README.md

@@ -18,8 +18,6 @@ The OpenTelemetry specification describes the cross-language requirements and ex
     - [Propagators](specification/context/api-propagators.md)
   - [Tracing](specification/trace/api.md)
   - [Metrics](specification/metrics/api.md)
-    - [User-Facing API](specification/metrics/api-user.md)
-    - [Meter API](specification/metrics/api-meter.md)
 - SDK Specification
   - [Tracing](specification/trace/sdk.md)
   - [Resource](specification/resource/sdk.md)

specification/glossary.md

@@ -40,17 +40,7 @@ Synonyms: *Instrumentation Library*, *Integration*.
 ### Tracer Name / Meter Name
 
 This refers to the `name` and (optional) `version` arguments specified when
-creating a new `Tracer` or `Meter` (see [Obtaining a Tracer](trace/api.md#obtaining-a-tracer)/[Obtaining a Meter](metrics/api-user.md#obtaining-a-meter)). It identifies the [Instrumenting Library](#instrumenting_library).
-
-### Namespace
-
-This term applies to [Metric names](metrics/api-user.md#metric-names) only. The namespace is used to disambiguate identical metric
-names used in different [instrumenting libraries](#instrumenting_library). The [Name](#name) provided
-for creating a `Meter` serves as its namespace in addition to the primary semantics
-described [here](#name).
-
-The `version` argument is not relevant here and will not be included in
-the resulting namespace string.
+creating a new `Tracer` or `Meter` (see [Obtaining a Tracer](trace/api.md#obtaining-a-tracer)/[Obtaining a Meter](metrics/api.md#meter-interface)). It identifies the [Instrumenting Library](#instrumenting_library).
 
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