[cart] document instrumentation (#390)

* cart service doc details

* clean up docs

Co-authored-by: Carter Socha <43380952+cartersocha@users.noreply.github.com>

docs/services/cartservice.md

@@ -0,0 +1,69 @@
+# cart service
+
+This service maintains items placed in the shopping cart by users. It interacts
+with a Redis caching service for fast access to shopping cart data.
+
+[Cart service source](../../src/cartservice/)
+
+## SDK Initialization
+
+The OpenTelemetry .NET SDK should be initialized in your application's
+`Startup.cs` as part of the `ConfigureServices` function. When initializing,
+optionally specify which instrumentation libraries to leverage. The SDK is
+initialized using a builder pattern, where you add each instrumentation library
+(with options), and the OTLP Exporter to be used. The SDK will make use of
+OpenTelemetry standard environment variables to configure the export endpoints,
+resource attributes, and service name.
+
+```cs
+    services.AddOpenTelemetryTracing((builder) => builder
+        .AddRedisInstrumentation(
+            cartStore.GetConnection(),
+            options => options.SetVerboseDatabaseStatements = true)
+        .AddAspNetCoreInstrumentation()
+        .AddGrpcClientInstrumentation()
+        .AddHttpClientInstrumentation()
+        .AddOtlpExporter());
+```
+
+## Traces
+
+OpenTelemetry Tracing in .NET, leverages the existing `Activity` classes as
+part of the core runtime.
+
+### Add attributes to auto-instrumented spans
+
+Within the execution of auto-instrumented code you can get current span
+(activity) from context.
+
+```cs
+    var activity = Activity.Current;
+```
+
+Adding attributes to a span (activity) is accomplished using `SetTag` on the
+activity object. In the `AddItem` function from `services/CartService.cs`
+several attributes are added to the auto-instrumented span.
+
+```cs
+    activity?.SetTag("app.user.id", request.UserId);
+    activity?.SetTag("app.product.quantity", request.Item.Quantity);
+    activity?.SetTag("app.product.id", request.Item.ProductId);
+```
+
+### Add span events
+
+Adding span (activity) events is accomplished using `AddEvent` on the activity
+object. In the `GetCart` function from `services/CartService.cs` a span event is
+added.
+
+```cs
+    activity?.AddEvent(new("Fetch cart"));
+```
+
+## Metrics
+
+TBD
+
+## Logs
+
+TBD