Make Debug OTLP export opt-in via FSHARP_OTEL_EXPORT

[xperiandri]

Problem

FSharpPackage's Debug-only constructor unconditionally starts an OTLP trace/metric exporter against a hardcoded http://127.0.0.1:4317. When nothing is listening there — the common case when just debugging the extension, without Jaeger/Prometheus running in a container — this is silent noise and a stall:

• the metric exporter retries against the closed port every second for the whole session;
• Dispose calls tracerProvider.ForceFlush(5000), so closing the experimental VS instance blocks for up to 5 seconds every time.

There was no way to turn this off short of editing the source and rebuilding.

Fix

FSharpServiceTelemetry.otelExport now only starts the OTLP exporter when FSHARP_OTEL_EXPORT is set to an absolute http/https URI with a non-empty host. A value like localhost:4317 parses as an absolute URI too (scheme localhost, no host), so it's treated the same as an empty or unparseable value and falls back to the historical default endpoint rather than being handed to the exporter as-is.

The default VisualFSharpDebug launch profile explicitly clears FSHARP_OTEL_EXPORT (rather than just omitting it), so a first F5 never surprises anyone with OTEL noise or the shutdown stall — including if the variable happened to be set in the ambient shell environment devenv.exe inherits from. A new OTEL Export profile in the same launchSettings.json sets it to opt back in — pick it from the run-button dropdown, no rebuild needed.

DEVGUIDE.md documents the switch next to the existing F5 instructions, and the VS release notes get an entry.

Testing

• dotnet fantomas --check on the changed .fs file.
• The endpoint-selection logic (unset/empty → no export; 1/localhost:4317 → default endpoint; a real http/https URI → used as-is) was exercised standalone via dotnet fsi against a stand-in copy of the logic.

🤖 Generated with Claude Code

[github-actions]

❗ Release notes required

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---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`vsintegration/src`	docs/release-notes/.VisualStudio/18.vNext.md

[xperiandri]

/azp run

WindowsCompressedMetadata transparent_compiler_release reports canceled, not a real failure: FSharp.Compiler.ComponentTests finished green at 10:55:46 (+8706/x0/?247), then the job produced no further log output for 74 minutes until AzDo force-canceled it on the overall job timeout. Unrelated to this PR's diff (vsintegration debug helper, launchSettings.json, docs only). Re-running.

[azure-pipelines]

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[xperiandri]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 20468 in repo dotnet/fsharp

[T-Gro]

🤖🕵️⏱️🔥

{"FSHARP_OTEL_EXPORT": ""}

Default F5 must opt out. Use an explicit export profile and clear inherited values in No OTEL.

[xperiandri]

Agreed. Flipped it: VisualFSharpDebug is now the default and explicitly clears FSHARP_OTEL_EXPORT (""), so an ambient value in the shell devenv.exe inherits from can't leak through either. Export moved to a new, explicitly-named OTEL Export profile.

[T-Gro]

🤖🕵️

$env:FSHARP_OTEL_EXPORT = 'localhost:4317'

UriKind.Absolute accepts this. Require http/https and a non-empty host.

[xperiandri]

Good catch — confirmed Uri.TryCreate("localhost:4317", UriKind.Absolute) returns true with Scheme = "localhost" and an empty Host. Added a guard requiring http/https and a non-empty host; anything else (including this case) now falls back to the default endpoint instead of being handed to the exporter.