Specify MeterProvider impl creation in SDK doc (open-telemetry#3067)

Currently the metric specification defines a normative recommendation
for API implementations in the API specification. The API specification
should be scope to specify the API, not the implementation. Instead move
this recommendation to the SDK specification where the implementation of
the API should be specified.

specification/metrics/api.md

@@ -105,12 +105,6 @@ Normally, the `MeterProvider` is expected to be accessed from a central place.
 Thus, the API SHOULD provide a way to set/register and access a global default
 `MeterProvider`.
 
-Notwithstanding any global `MeterProvider`, some applications may want to or
-have to use multiple `MeterProvider` instances, e.g. to have different
-configuration for each, or because its easier with dependency injection
-frameworks. Thus, implementations of `MeterProvider` SHOULD allow creating an
-arbitrary number of `MeterProvider` instances.
-
 ### MeterProvider operations
 
 The `MeterProvider` MUST provide the following functions:

specification/metrics/sdk.md

@@ -12,6 +12,7 @@ linkTitle: SDK
 <!-- toc -->
 
 - [MeterProvider](#meterprovider)
+  * [MeterProvider Creation](#meterprovider-creation)
   * [Meter Creation](#meter-creation)
   * [Shutdown](#shutdown)
   * [ForceFlush](#forceflush)
@@ -74,6 +75,10 @@ metrics produced by any `Meter` from the `MeterProvider`. The [tracing SDK
 specification](../trace/sdk.md#additional-span-interfaces) has provided some
 suggestions regarding how to implement this efficiently.
 
+### MeterProvider Creation
+
+The SDK SHOULD allow the creation of multiple independent `MeterProvider`s.
+
 ### Meter Creation
 
 New `Meter` instances are always created through a `MeterProvider`