Stop health check probes filling the request log

src/SchoolAccount.Collect.Api/Extensions/MiddlewareExtensions.cs

@@ -1,4 +1,6 @@
-using SchoolAccount.Collect.Api.Middleware;
+using SchoolAccount.Collect.Api.Middleware;
+using Serilog;
+using Serilog.Events;
 
 namespace SchoolAccount.Collect.Api.Extensions;
 
@@ -10,4 +12,37 @@ public static IApplicationBuilder UseRequestContextLogging(this IApplicationBuil
 
         return app;
     }
+
+    public static IApplicationBuilder UseRequestLogging(this IApplicationBuilder app)
+    {
+        app.UseSerilogRequestLogging(options => options.GetLevel = GetRequestLogLevel);
+
+        return app;
+    }
+
+    // Decides how loudly to record each request summary.
+    //
+    // The container platform polls the health endpoint constantly, and a summary line per probe
+    // buries everything else in the log stream. Those drop to Verbose, which nothing collects,
+    // so they cost nothing but can be switched on if the probes themselves need investigating.
+    //
+    // Everything else keeps Serilog's own defaults: failures at Error, ordinary requests at
+    // Information. The status code is checked as well as the exception because the global
+    // exception handler deals with most failures before they reach this middleware, so a 500
+    // usually arrives here with no exception attached.
+    private static LogEventLevel GetRequestLogLevel(
+        HttpContext context,
+        double elapsedMilliseconds,
+        Exception? exception
+    )
+    {
+        if (exception is not null || context.Response.StatusCode >= 500)
+        {
+            return LogEventLevel.Error;
+        }
+
+        return context.Request.Path.StartsWithSegments("/health")
+            ? LogEventLevel.Verbose
+            : LogEventLevel.Information;
+    }
 }

src/SchoolAccount.Collect.Api/Program.cs

@@ -38,7 +38,7 @@
 
 app.UseRequestContextLogging();
 
-app.UseSerilogRequestLogging();
+app.UseRequestLogging();
 
 app.UseExceptionHandler();