Update default host and port for PrometheusExporter to align with the OTel spec. (#2783)

Author: Yun-Ting

src/OpenTelemetry.Exporter.Prometheus/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+* Update default `httpListenerPrefixes` for PrometheusExporter to be `http://localhost:9464/`.
+([2783](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2783))
+
 ## 1.2.0-rc1
 
 Released 2021-Nov-29

src/OpenTelemetry.Exporter.Prometheus/PrometheusExporterOptions.cs

@@ -29,7 +29,7 @@ public class PrometheusExporterOptions
         internal Func<DateTimeOffset> GetUtcNowDateTimeOffset = () => DateTimeOffset.UtcNow;
 
         private int scrapeResponseCacheDurationMilliseconds = 10 * 1000;
-        private IReadOnlyCollection<string> httpListenerPrefixes = new string[] { "http://*:80/" };
+        private IReadOnlyCollection<string> httpListenerPrefixes = new string[] { "http://localhost:9464/" };
 
 #if NETCOREAPP3_1_OR_GREATER
         /// <summary>
@@ -47,7 +47,7 @@ public class PrometheusExporterOptions
 
         /// <summary>
         /// Gets or sets the prefixes to use for the http listener. Default
-        /// value: http://*:80/.
+        /// value: http://localhost:9464/.
         /// </summary>
         public IReadOnlyCollection<string> HttpListenerPrefixes
         {

src/OpenTelemetry.Exporter.Prometheus/README.md

@@ -83,7 +83,7 @@ properties:
 
 * `HttpListenerPrefixes`: Defines the prefixes which will be used by the
   listener when `StartHttpListener` is `true`. The default value is
-  `["http://*:80/"]`. You may specify multiple endpoints.
+  `["http://localhost:9464/"]`. You may specify multiple endpoints.
 
   For details see:
   [HttpListenerPrefixCollection.Add(String)](https://docs.microsoft.com/dotnet/api/system.net.httplistenerprefixcollection.add)