Add better documentation for IResourceDetector and OltpExporter confi…

…guration (#4230)

Co-authored-by: Cijo Thomas <[email protected]>
Co-authored-by: Mikel Blanchard <[email protected]>

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/README.md

@@ -67,7 +67,16 @@ and environment variables.
 The `OtlpExporterOptions` type setters take precedence over the environment variables.
 
 This can be achieved by providing an `Action<OtlpExporterOptions>` delegate to the
-`AddOtlpExporter()` method.
+`AddOtlpExporter()` method or using `AddOptions<OtlpExporterOptions>()`.
+
+If additional services from the dependency injection are required, they can be
+configured like this:
+
+```csharp
+services.AddOptions<OtlpExporterOptions>().Configure<Service>((opts, svc) => {
+    // ...
+});
+```
 
 TODO: Show metrics specific configuration (i.e MetricReaderOptions).
 

src/OpenTelemetry.Extensions.Hosting/README.md

@@ -83,6 +83,41 @@ app.Run();
 A fully functional example can be found
 [here](https://github.com/open-telemetry/opentelemetry-dotnet/tree/main/examples/AspNetCore).
 
+### Resources
+
+To dynamically add resources at startup from the dependency injection you can
+provide an `IResourceDetector`.
+To make use of it add it to the dependency injection and they you can use the
+`ISerivceProvider` add it to OpenTelemetry:
+
+```csharp
+public class MyResourceDetector : IResourceDetector
+{
+    private readonly IWebHostEnvironment webHostEnvironment;
+
+    public MyResourceDetector(IWebHostEnvironment webHostEnvironment)
+    {
+        this.webHostEnvironment = webHostEnvironment;
+    }
+
+    public Resource Detect()
+    {
+        return ResourceBuilder.CreateEmpty()
+            .AddService(serviceName: this.webHostEnvironment.ApplicationName)
+            .AddAttributes(new Dictionary<string, object> { ["host.environment"] = this.webHostEnvironment.EnvironmentName })
+            .Build();
+    }
+}
+
+services.AddSingleton<MyResourceDetector>();
+
+services.AddOpenTelemetry()
+    .ConfigureResource(builder =>
+        builder.AddDetector(sp => sp.GetRequiredService<MyResourceDetector>()))
+    .WithTracing(builder => builder.AddConsoleExporter())
+    .WithMetrics(builder => builder.AddConsoleExporter());
+```
+
 ## Migrating from pre-release versions of OpenTelemetry.Extensions.Hosting
 
 Pre-release versions (all versions prior to 1.4.0) of