Remove default keys from metrics spec (open-telemetry#565)

specification/metrics/api.md

@@ -8,7 +8,6 @@
   * [Meter Interface](#meter-interface)
   * [Aggregations](#aggregations)
   * [Time](#time)
-  * [WithRecommendedKeys declaration on metric instruments](#withrecommendedkeys-declaration-on-metric-instruments)
   * [Metric Event Format](#metric-event-format)
 - [Three kinds of instrument](#three-kinds-of-instrument)
   * [Counter](#counter)
@@ -198,38 +197,6 @@ use of this term for the SDK specification, to refer to parts of a
 data format that express explicitly timestamped values, in a sequence,
 resulting from an aggregation of raw measurements over time.
 
-### WithRecommendedKeys declaration on metric instruments
-
-A standard feature of metric SDKs is to pre-aggregate metric events
-according to a specified set of label keys (i.e., dimensions).  To
-perform this task, the SDK must aggregate metric events over the
-collection interval: (1) across time, (2) across key dimensions in
-_label space_.
-
-When aggregating across spatial dimensions, metric events for different sets of
-labels are combined into an aggregated value for each distinct "group" of
-values for the key dimensions.  It means that measurements are combined for all
-metric events having the same values for selected keys, explicitly disregarding
-any additional labels with keys not in the set of aggregation keys.  Some
-exporters are known to require pre-specifying the label keys used for
-aggregation (e.g., Prometheus).
-
-For example, if `[ak1, ak2]` are the aggregation keys and `[ik1,
-ik2]` are the ignored keys, then a metric event having labels
-`{ak1=A, ak2=B, ik1=C, ik1=D}` will be combined with a metric
-event having labels `{ak1=A, ak2=B, ik1=Y, ik1=Z}` because they
-have identical label values for all of the aggregation keys.
-
-The API provides a `WithRecommendedKeys` option for the user to declare the
-recommended aggregation keys when declaring new metric instruments,
-intended as the default way to configure an exporter for
-pre-aggregation, if it is expected.  Since this is only expected in
-some exporters, it is regarded as an option relevant to the exporter,
-whether keys configured through `WithRecommendedKeys` are applied for aggregation
-purposes or not.  This allows the user to influence the standard
-implementation behavior, especially for exporters that require
-pre-specified aggregation keys.
-
 ### Metric Event Format
 
 Metric events have the same logical representation, regardless of