Add details to Prometheus Exporter requirement

specification/metrics/sdk_exporters/prometheus.md

@@ -2,6 +2,25 @@
 
 **Status**: [Experimental](../../document-status.md)
 
-Prometheus Exporter is a [Pull Metric Exporter](../sdk.md#pull-metric-exporter)
-which reacts to the Prometheus scraper and report the metrics passively to
-[Prometheus](https://prometheus.io/).
+Prometheus Exporter is a [MetricExporter](../sdk.md#metricexporter) which reacts
+to the Prometheus scraper and reports the metrics passively to
+[Prometheus](https://prometheus.io/), or [pushes
+metrics](https://prometheus.io/docs/instrumenting/pushing/) to a [Prometheus
+Pushgateway](https:/prometheus/pushgateway).
+
+Prometheus Exporter SHOULD support [Pull mode](../sdk.md#pull-metric-exporter),
+if possible.
+
+Support for [Push mode](../sdk.md#push-metric-exporter) is out of scope for the
+initial release.
+
+Prometheus Exporter MUST support version `0.0.4` of the [Text-based
+format](https:/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#text-based-format).
+
+Prometheus Exporter SHOULD only support [Cumulative
+Temporality](../datamodel.md#temporality).
+
+Prometheus Exporter MAY support [OpenMetrics Text
+Format](https:/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#openmetrics-text-format),
+including the
+[Exemplars](https:/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars).