Make prometheus host and port config options explicit (#4147)

Splitting off a portion of #4142. This PR makes the host and port
configuration options explicit for the prometheus exporter. Currently,
they're [implied from the corresponding env
vars](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md#prometheus-exporter)
`OTEL_EXPORTER_PROMETHEUS_HOST` and `OTEL_EXPORTER_PROMETHEUS_PORT`.

See this
[convo](https://github.com/open-telemetry/opentelemetry-specification/pull/4142/files#r1674675871)
for more details.

---------

Co-authored-by: Robert Pająk <[email protected]>
Co-authored-by: Trask Stalnaker <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -16,6 +16,9 @@ release.
 
 ### Metrics
 
+- Clarify prometheus exporter should have `host` and `port` configuration options.
+  ([#4147](https://github.com/open-telemetry/opentelemetry-specification/pull/4147))
+
 ### Logs
 
 ### Events

specification/metrics/sdk_exporters/prometheus.md

@@ -40,6 +40,14 @@ Prometheus exporter.
 
 ## Configuration
 
+A Prometheus Exporter SHOULD support a configuration option to set the host
+that metrics are served on. The option MAY be named `host`, and MUST be `localhost`
+by default.
+
+A Prometheus Exporter SHOULD support a configuration option to set the port
+that metrics are served on. The option MAY be named `port`, and MUST be `9464` by
+default.
+
 A Prometheus Exporter MAY offer configuration to add resource attributes as metric attributes.
 By default, it MUST NOT add any resource attributes as metric attributes.
 The configuration SHOULD allow the user to select which resource attributes to copy (e.g.