Make Debug OTLP export opt-in via FSHARP_OTEL_EXPORT

DEVGUIDE.md

@@ -374,6 +374,8 @@ Then, use the **f5** or **ctrl+f5** keyboard shortcuts to test your tooling chan
 > [!TIP]
 > If the F# extension fails to load when you F5 (because the Roslyn version in the repo is ahead of the one in your Visual Studio), launch VS with `.\start-vs-VisualFSharpSln.ps1` instead of opening the solution directly. It builds the extension against the Roslyn your VS actually ships, so you don't need an internal/nightly VS or a local Roslyn build. Requires VS with Roslyn 5.10 or newer.
 
+Debug builds export traces and metrics over OTLP only when `FSHARP_OTEL_EXPORT` is set, so the default `VisualFSharpDebug` profile stays quiet without a collector running. Pick the `OTEL Export` launch profile - it points `FSHARP_OTEL_EXPORT` at `http://127.0.0.1:4317` - to debug against Jaeger, Prometheus or another OTLP collector listening there.
+
 Alternatively, you can do this entirely via the command line if you prefer that:
 
 ```shell

docs/release-notes/.VisualStudio/18.vNext.md

@@ -22,3 +22,4 @@
 * Unused analyzers: disable in VS when file has errors ([PR #19892](https://github.com/dotnet/fsharp/pull/19892))
 * Move to Roslyn's unified ExternalAccess library ([PR #20099](https://github.com/dotnet/fsharp/pull/20099))
 * Remove trailing whitespace from source files. No functional change: whitespace inside string literals and inactive `#if` regions is preserved. ([PR #20355](https://github.com/dotnet/fsharp/pull/20355))
+* The Debug build no longer exports OpenTelemetry unless `FSHARP_OTEL_EXPORT` asks it to (the variable can also carry the collector endpoint), so the default `VisualFSharpDebug` launch profile no longer stalls or retries against a missing collector; the new `OTEL Export` profile turns it back on. ([PR #20468](https://github.com/dotnet/fsharp/pull/20468))

vsintegration/Vsix/VisualFSharpFull/Properties/launchSettings.json

@@ -11,7 +11,23 @@
         "FSharp_Shim_Present": "true",
         "FSharpPreferAnyCpuTools": "true",
         "Fsc_NetFramework_ToolPath": "$(FSharpCompilerPathForDebuggingLocally)",
-        "Fsc_NetFramework_AnyCpu_ToolExe": "fscAnyCpu.exe"
+        "Fsc_NetFramework_AnyCpu_ToolExe": "fscAnyCpu.exe",
+        "FSHARP_OTEL_EXPORT": ""
+      }
+    },
+    "OTEL Export": {
+      "commandName": "Executable",
+      "executablePath": "$(DevEnvDir)devenv.exe",
+      "commandLineArgs": "/rootsuffix $(VSSDKTargetPlatformRegRootSuffix) /log",
+      "environmentVariables": {
+        "FSharpCompilerPath": "$(FSharpCompilerPathForDebuggingLocally)",
+        "DisableAutoSetFscCompilerPath": "true",
+        "FSharpPreferNetFrameworkTools": "true",
+        "FSharp_Shim_Present": "true",
+        "FSharpPreferAnyCpuTools": "true",
+        "Fsc_NetFramework_ToolPath": "$(FSharpCompilerPathForDebuggingLocally)",
+        "Fsc_NetFramework_AnyCpu_ToolExe": "fscAnyCpu.exe",
+        "FSHARP_OTEL_EXPORT": "http://127.0.0.1:4317"
       }
     }
   }

vsintegration/src/FSharp.Editor/Common/DebugHelpers.fs

@@ -136,10 +136,26 @@ module FSharpServiceTelemetry =
     open OpenTelemetry.Trace
     open OpenTelemetry.Metrics
 
-    let otelExport () =
-        // On Windows forwarding localhost to wsl2 docker container sometimes does not work. Use IP address instead.
-        let otlpEndpoint = Uri("http://127.0.0.1:4317")
-
+    // Nothing listening on the endpoint means an exporter retrying in the background and a 5s flush
+    // on shutdown, so exporting is opt-in: FSHARP_OTEL_EXPORT enables it and can carry the endpoint.
+    let private otelEndpoint =
+        match Environment.GetEnvironmentVariable "FSHARP_OTEL_EXPORT" with
+        | null
+        | "" -> ValueNone
+        | value ->
+            match Uri.TryCreate(value, UriKind.Absolute) with
+            | true, uri when
+                (uri.Scheme = Uri.UriSchemeHttp || uri.Scheme = Uri.UriSchemeHttps)
+                && not (String.IsNullOrEmpty uri.Host)
+                ->
+                ValueSome uri
+            // "localhost:4317" parses as an absolute URI too - scheme "localhost", no host - so an
+            // http(s) scheme and a host are required; anything else falls back to the default, same as
+            // an unparseable value. On Windows forwarding localhost to wsl2 docker sometimes does not
+            // work, so that default is an IP address rather than the name.
+            | _ -> ValueSome(Uri "http://127.0.0.1:4317")
+
+    let private startOtelExport (otlpEndpoint: Uri) =
         let meterProvider =
             // Configure OpenTelemetry metrics. Metrics can be viewed in Prometheus or other compatible tools.
             OpenTelemetry.Sdk
@@ -170,5 +186,10 @@ module FSharpServiceTelemetry =
             tracerProvider.Dispose()
             meterProvider.Dispose()
 
+    let otelExport () =
+        match otelEndpoint with
+        | ValueNone -> ignore
+        | ValueSome endpoint -> startOtelExport endpoint
+
     let listenToAll () = listen ""
 #endif