Clarify that attribute keys are unique in collections

Attributes keys must be unique. The key/value pair collections in the specification
was always intended to model a map.

There was a recent confusion about this. This change clarifies the spec.

Resolves open-telemetry#2245

specification/common/common.md

@@ -105,3 +105,14 @@ attribute limits for Resources.
 Attributes, which belong to Metrics, are exempt from the limits described above
 at this time, as discussed in
 [Metrics Attribute Limits](../metrics/sdk.md#attribute-limits).
+
+## Attribute Collections
+
+[Resources](../resource/sdk.md), Metrics
+[data points](../metrics/datamodel.md#metric-points),
+[Spans](../trace/api.md#set-attributes), Span
+[Events](../trace/api.md#add-events), Span
+[Links](../trace/api.md#specifying-links) and
+[Log Records](../logs/data-model.md) may contain a collection of attributes. The
+keys in each such collection are unique, i.e. there cannot exist more than one
+key-value pair with the same key.