add support for exemplars on OpenMetrics counters (open-telemetry#2483)

specification/metrics/datamodel.md

@@ -1192,8 +1192,10 @@ Prometheus Cumulative metrics do not include the start time of the metric. When
 #### Exemplars
 
 [OpenMetrics Exemplars](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars)
-can be attached to Prometheus Histogram bucket metrics, which SHOULD be
-converted to exemplars on OpenTelemetry histograms. If present, the timestamp
+can be attached to Prometheus Histogram bucket metric points and counter metric
+points. Exemplars on histogram buckets SHOULD be converted to exemplars on
+OpenTelemetry histograms. Exemplars on counter metric points SHOULD be
+converted to exemplars on OpenTelemetry sums. If present, the timestamp
 MUST be added to the OpenTelemetry exemplar. The Trace ID and Span ID SHOULD be
 retrieved from the `trace_id` and `span_id` label keys, respectively.  All
 labels not used for the trace and span ids MUST be added to the OpenTelemetry
@@ -1305,14 +1307,14 @@ OpenTelemetry Metric Attributes MUST be converted to [Prometheus labels](https:/
 
 #### Exemplars
 
-[Exemplars](#exemplars) on OpenTelemetry Histograms SHOULD be converted to
-OpenMetrics exemplars. Exemplars on other OpenTelemetry data points MUST be
-dropped. For Prometheus push exporters, multiple exemplars are able to be
-added to each bucket, so all exemplars SHOULD be converted. For Prometheus
-pull endpoints, only a single exemplar is able to be added to each bucket, so
-the largest exemplar from each bucket MUST be used, if attaching exemplars. If
-no exemplars exist on a bucket, the highest exemplar from a lower bucket MUST
-be used, even though it is a duplicate of another bucket's exemplar.
+[Exemplars](#exemplars) on OpenTelemetry Histograms and Monotonic Sums SHOULD
+be converted to OpenMetrics exemplars. Exemplars on other OpenTelemetry data
+points MUST be dropped. For Prometheus push exporters, multiple exemplars are
+able to be added to each bucket, so all exemplars SHOULD be converted. For
+Prometheus pull endpoints, only a single exemplar is able to be added to each
+bucket, so the largest exemplar from each bucket MUST be used, if attaching
+exemplars. If no exemplars exist on a bucket, the highest exemplar from a lower
+bucket MUST be used, even though it is a duplicate of another bucket's exemplar.
 OpenMetrics Exemplars MUST use the `trace_id` and `span_id` keys for the trace
 and span IDs, respectively. Timestamps MUST be added as timestamps on the
 OpenMetrics exemplar, and `filtered_attributes` MUST be added as labels on the