Expose ExemplarReservoir as configuration option of View. (open-telemetry#3820)

Fixes open-telemetry#2922
Fixes open-telemetry#3812 
Related to open-telemetry#3756

## Changes

- Cleans up language around exposing `ExemplarReservoir`. (Remove TODO,
e.g.)
- Remove `ExemplarFilter` interface but keep configuration options. (see
open-telemetry#3812)
- Clarify / simplify Spec Compliance matrix for existing state of the
Exemplar Specification.


Prototype in java:
open-telemetry/opentelemetry-java#5960

Author: jsuereth

CHANGELOG.md

@@ -13,6 +13,11 @@ release.
 
 ### Metrics
 
+- Expose `ExemplarReservoir` as configuration parameter for views.
+  Remove `ExemplarFilter` as an interface, now it is only configuration
+  parameter.
+  ([#3820](https://github.com/open-telemetry/opentelemetry-specification/pull/3820))
+
 ### Logs
 
 - Fix `Resource` field type in Logs Data Model.