Added example of TracesSampler callback function to MVC Sample (#3593)

Author: jamescrosswell

samples/Sentry.Samples.AspNetCore.Mvc/Controllers/HomeController.cs

@@ -13,6 +13,15 @@ public IActionResult Index()
         return View();
     }
 
+    [HttpGet("[controller]/sampler")]
+    public Task<string> Sampler()
+    {
+        // The sampling for this route is determined by the TraceSampler function configured in Program.cs when
+        // initializing the Sentry SDK... here we just display the result of that sampling decision.
+        var transaction = SentrySdk.GetSpan()?.GetTransaction();
+        return Task.FromResult($"Sampled: {transaction?.IsSampled}");
+    }
+
     // GET /home/block/true or /home/block/false to observe events
     [HttpGet("[controller]/block/{block?}")]
     public async Task<string> Block([FromRoute] bool block)

samples/Sentry.Samples.AspNetCore.Mvc/Program.cs

@@ -1,5 +1,6 @@
 using System.Net;
 using Samples.AspNetCore.Mvc;
+using Sentry.AspNetCore;
 using Sentry.Extensibility;
 
 var builder = WebApplication.CreateBuilder(args);
@@ -17,8 +18,6 @@
 
         options.MaxBreadcrumbs = 200;
 
-        options.TracesSampleRate = 1.0;
-
         // Set a proxy for outgoing HTTP connections
         options.HttpProxy = null; // new WebProxy("https://localhost:3128");
 
@@ -31,7 +30,33 @@
         options.MaxQueueItems = 100;
         options.ShutdownTimeout = TimeSpan.FromSeconds(5);
 
-        options.TracesSampleRate = 1.0; // For production you may want to lower this to stay inside your quota
+        options.TracesSampleRate = 1.0; // For production, you may want to lower this to stay inside your quota
+
+        // In addition to (or instead of) setting the TracesSampleRate to a fixed value, you can also define a function
+        // that calculates the sample rate dynamically based on the current context.
+        options.TracesSampler = ctx =>
+        {
+            // In distributed tracing scenarios, we may want to respect upstream sampling decisions... for example, if
+            // the javascript frontend contains tracing instrumentation, a decision not to sample this request may
+            // already have been made. In that case, we don't want to sample this request since it would result in an
+            // incomplete trace.
+            if (ctx.TransactionContext.IsParentSampled == false)
+            {
+                // If the parent transaction is not sampled, don't sample this one either
+                return 0.0;
+            }
+
+            // Only sample 30% of the requests to this /home/sampler route
+            var httpPath = ctx.TryGetHttpPath();
+            if (string.Equals(httpPath, "/home/sampler", StringComparison.OrdinalIgnoreCase))
+            {
+                return 0.3;
+            }
+
+            // For all other routes, use the statically configured sample rate. Returning null here would achieve the
+            // same thing (if the TraceSampler function returns null, the SDK falls back to the TracesSampleRate)
+            return options.TracesSampleRate;
+        };
 
         // Configures the root scope
         options.ConfigureScope(s => s.SetTag("Always sent", "this tag"));