Add details to Prometheus Exporter requirement

specification/metrics/sdk_exporters/prometheus.md

@@ -2,6 +2,32 @@
 
 **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.
+
+Prometheus Exporter MAY support [Push mode](../sdk.md#push-metric-exporter), if
+there is a need to support short-lived applications (e.g. short-lived
+service-level batch jobs, JavaScript applications running inside web browsers,
+etc.).
+
+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). Attempt to change the [Aggregation
+Temporality](../datamodel.md#temporality) to
+[Delta](../datamodel.md#temporality) SHOULD be treated as an error. It is
+unspecified _how_ the this error should be handled (e.g. it could fail fast
+during the exporter configuration time). Please refer to [Error handling in
+OpenTelemetry](../../error-handling.md) for the general guidance.
+
+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).