Add details to Prometheus Exporter requirement

CHANGELOG.md

@@ -17,6 +17,8 @@ release.
   ([#2101](/~https://github.com/open-telemetry/opentelemetry-specification/pull/2101))
 - Mark In-memory, OTLP and Stdout exporter specs as Feature-freeze.
   ([#2131](/~https://github.com/open-telemetry/opentelemetry-specification/pull/2131))
+- Add details to the Prometheus Exporter requirements.
+  ([#2124](/~https://github.com/open-telemetry/opentelemetry-specification/pull/2124))
 - Remove the concept of supported temporality, keep preferred.
   ([#2154](/~https://github.com/open-telemetry/opentelemetry-specification/pull/2154))
 - Mark In-memory, OTLP and Stdout exporter specs as Stable.

specification/metrics/sdk_exporters/prometheus.md

@@ -2,6 +2,22 @@
 
 **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/).
+A Prometheus Exporter is a [Pull Metric Exporter](../sdk.md#pull-metric-exporter)
+which reports metrics by responding to the [Prometheus](https://prometheus.io/)
+scraper requests.
+
+A Prometheus Exporter MUST support [Pull mode](../sdk.md#pull-metric-exporter).
+
+A Prometheus Exporter MUST NOT support [Push
+mode](../sdk.md#push-metric-exporter).
+
+A Prometheus Exporter MUST only support [Cumulative
+Temporality](../datamodel.md#temporality).
+
+A Prometheus Exporter MUST support version `0.0.4` of the [Text-based
+format](/~https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#text-based-format).
+
+A Prometheus Exporter MAY support [OpenMetrics Text
+Format](/~https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#openmetrics-text-format),
+including the
+[Exemplars](/~https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars).