Metrics API: Rename Cumulative to Counter

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+# Rename "Cumulative" to "Counter" in the metrics API
+
+**Status:** `proposed`
+
+Prefer the name "Counter" as oppose to "Cumulative".
+
+## Motivation
+
+Informally speaking, it seems that OpenTelemetry community members would prefer to call Cumulative metric instruments "Counters".  During conversation (e.g., in the 8/21 working session), this has become clear.
+
+Counter is a noun, like the other kinds Gauge and Measure.  Cumulative is an adjective, so while "Cumulative instrument" makes sense, it describes a "Counter".
+
+## Explanation
+
+This will eliminate the cognitive cost of mapping "counter" to "cumulative" when speaking about these APIs.
+
+## Internal details
+
+Simply replace every "Cumulative" with "Counter", then edit for grammar. 
+
+## Prior art and alternatives
+
+In a survey of existing metrics libraries, Counter is far more common.