Optimization: HostWarmupMiddleware (JIT profile fix and async throughput) (#8588)

This is from some worker profiling and noticed the JIT trace here was running each invocation which looked unintended - confirmed with @safihamid. This now runs for `JitTraceRuntime` just once per instance of the middleware (effectively: per host, but if we _did_ change the middleware ever it could re-run for future flexibility). This also means the profile it creates on disk would be the _last_ instead of the _first_. Whether these differ at all would depend on how busy the stamp is.

While in here, also removed a few allocations and the async overhead of the middleware for _all_ requests (whether in warmup state or not). It's not much, but about a 1% throughput gain from lower latency per request.

#### Benchmarking
```powershell
.\bombard.exe -k -l -c 100 -d 60s "http://localhost:5000/api/HttpTriggerAnon?name=bob"
```

#### Before (~61,050 rps)
```lua
Statistics        Avg      Stdev        Max
  Reqs/sec     61336.81   13051.80  131179.96
  Latency        1.63ms   427.90us   317.42ms
  Latency Distribution
     50%     1.51ms
     75%     2.00ms
     90%     2.00ms
     95%     2.06ms
     99%     5.00ms
  HTTP codes:
    1xx - 0, 2xx - 3677932, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    14.67MB/s

Statistics        Avg      Stdev        Max
  Reqs/sec     61108.92   12781.70   76012.83
  Latency        1.63ms   395.52us    66.31ms
  Latency Distribution
     50%     1.51ms
     75%     2.00ms
     90%     2.00ms
     95%     2.07ms
     99%     5.04ms
  HTTP codes:
    1xx - 0, 2xx - 3665847, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    14.62MB/s

Statistics        Avg      Stdev        Max
  Reqs/sec     60706.31   12939.59   77057.50
  Latency        1.64ms   433.08us    74.49ms
  Latency Distribution
     50%     1.51ms
     75%     2.00ms
     90%     2.00ms
     95%     2.09ms
     99%     5.00ms
  HTTP codes:
    1xx - 0, 2xx - 3641578, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    14.53MB/s
```


#### After (~62,310 rps)
```lua
Statistics        Avg      Stdev        Max
  Reqs/sec     61925.27   13209.73   79099.13
  Latency        1.61ms   451.98us    99.54ms
  Latency Distribution
     50%     1.51ms
     75%     2.00ms
     90%     2.01ms
     95%     2.21ms
     99%     5.00ms
  HTTP codes:
    1xx - 0, 2xx - 3715842, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    14.82MB/s

Statistics        Avg      Stdev        Max
  Reqs/sec     62510.94   12910.73   81487.14
  Latency        1.60ms   405.25us    99.18ms
  Latency Distribution
     50%     1.51ms
     75%     2.00ms
     90%     2.01ms
     95%     2.21ms
     99%     4.53ms
  HTTP codes:
    1xx - 0, 2xx - 3749532, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    14.96MB/s

Statistics        Avg      Stdev        Max
  Reqs/sec     62496.97   12958.93   78327.32
  Latency        1.60ms   394.55us    96.56ms
  Latency Distribution
     50%     1.50ms
     75%     2.00ms
     90%     2.00ms
     95%     2.05ms
     99%     5.00ms
  HTTP codes:
    1xx - 0, 2xx - 3748852, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    14.96MB/s
```

### Pull request checklist

* [x] My changes **do not** require documentation changes
    * [ ] Otherwise: Documentation issue linked to PR
* [x] My changes **should not** be added to the release notes for the next release
    * [ ] Otherwise: I've added my notes to `release_notes.md`
* [ ] My changes **do not** need to be backported to a previous version
    * [ ] Otherwise: Backport tracked by issue/PR #issue_or_pr
* [x] My changes **do not** require diagnostic events changes
    * Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
* [ ] I have added all required tests (Unit tests, E2E tests)

Author: NickCraver

src/WebJobs.Script.WebHost/Middleware/HostWarmupMiddleware.cs

@@ -22,6 +22,10 @@ public class HostWarmupMiddleware
         private readonly IScriptHostManager _hostManager;
         private readonly ILogger _logger;
         private string _assemblyLocalPath;
+        private volatile bool _jitTraceHasRun;
+
+        private static readonly PathString _warmupRoutePath = new PathString($"/api/{WarmUpConstants.FunctionName}");
+        private static readonly PathString _warmupRouteAlternatePath = new PathString($"/api/{WarmUpConstants.AlternateRoute}");
 
         public HostWarmupMiddleware(RequestDelegate next, IScriptWebHostEnvironment webHostEnvironment, IEnvironment environment, IScriptHostManager hostManager, ILogger<HostWarmupMiddleware> logger)
         {
@@ -33,20 +37,35 @@ public HostWarmupMiddleware(RequestDelegate next, IScriptWebHostEnvironment webH
             _assemblyLocalPath = Path.GetDirectoryName(new Uri(typeof(HostWarmupMiddleware).Assembly.Location).LocalPath);
         }
 
-        public async Task Invoke(HttpContext httpContext)
+        public Task Invoke(HttpContext httpContext)
         {
             if (IsWarmUpRequest(httpContext.Request, _webHostEnvironment.InStandbyMode, _environment))
+            {
+                return WarmupInvoke(httpContext);
+            }
+
+            return _next.Invoke(httpContext);
+        }
+
+        /// <summary>
+        /// This is so we only pay the async overhead while in the warmup path, but not for primary runtime.
+        /// </summary>
+        public async Task WarmupInvoke(HttpContext httpContext)
+        {
+            // We only want to run our JIT traces on the first warmup call.
+            if (!_jitTraceHasRun)
             {
                 PreJitPrepare(WarmUpConstants.JitTraceFileName);
                 if (_environment.IsLinuxConsumption())
                 {
                     PreJitPrepare(WarmUpConstants.LinuxJitTraceFileName);
                 }
+                _jitTraceHasRun = true;
+            }
 
-                ReadRuntimeAssemblyFiles();
+            ReadRuntimeAssemblyFiles();
 
-                await WarmUp(httpContext.Request);
-            }
+            await WarmUp(httpContext.Request);
 
             await _next.Invoke(httpContext);
         }
@@ -127,10 +146,9 @@ public async Task WarmUp(HttpRequest request)
 
         public static bool IsWarmUpRequest(HttpRequest request, bool inStandbyMode, IEnvironment environment)
         {
-            return inStandbyMode &&
-                ((environment.IsAppService() && request.IsAppServiceInternalRequest(environment)) || environment.IsLinuxConsumption()) &&
-                (request.Path.StartsWithSegments(new PathString($"/api/{WarmUpConstants.FunctionName}")) ||
-                request.Path.StartsWithSegments(new PathString($"/api/{WarmUpConstants.AlternateRoute}")));
+            return inStandbyMode
+                && ((environment.IsAppService() && request.IsAppServiceInternalRequest(environment)) || environment.IsLinuxConsumption())
+                && (request.Path.StartsWithSegments(_warmupRoutePath) || request.Path.StartsWithSegments(_warmupRouteAlternatePath));
         }
     }
 }