prometheus exporter SHOULD be modeled as a Collector

specification/metrics/sdk_exporters/prometheus.md

@@ -14,11 +14,16 @@ OpenTelemetry metrics MUST be converted to Prometheus metrics according to the
 
 A Prometheus Exporter SHOULD use
 [Prometheus client libraries](https://prometheus.io/docs/instrumenting/clientlibs/)
-for serving Prometheus metrics, including negotiating the
-[format](https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#openmetrics-text-format)
-of the response using the Content-Type header. Regardless of whether a
-Prometheus client library is used, the 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).
+for serving Prometheus metrics. This allows the prometheus client to negotiate
+the [format](https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md)
+of the response using the `Content-Type` header. If a prometheus client library
+is used, the OpenTelemetry Prometheus Exporter SHOULD be modeled as a
+[custom Collector](https://prometheus.io/docs/instrumenting/writing_clientlibs/#overall-structure)
+so it can be used in conjunction with existing Prometheus instrumentation.
+
+Regardless of whether a Prometheus client library is used, the 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 Exemplars and Exponential Histograms,
 which are [not currently supported by the Prometheus text format](../../compatibility/prometheus_and_openmetrics.md#differences-between-prometheus-formats),
 by supporting other Prococols, but is not required to implement them.