Make OTel export opt-in via FSHARP_OTEL_EXPORT

[xperiandri]

otelExport() unconditionally started exporting metrics and traces to http://127.0.0.1:4317. With nothing listening there — the normal case — that is a background exporter stuck retrying and a 5s flush on every shutdown, for every Debug VSIX session regardless of whether anyone is collecting the data.

Exporting now only starts when FSHARP_OTEL_EXPORT names a collector endpoint. The VisualFSharpDebug launch profile sets it so F5 behaves as before, and a new "No OTEL" profile omits it.

The code is under #if DEBUG, so this only affects Debug VSIX builds.

🤖 Generated with Claude Code

[github-actions]

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@xperiandri,

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[xperiandri]

Duplicate of #20468, which does the same thing more completely (DEVGUIDE, release notes, and a dedicated OTEL Export launch profile). Closing in favour of that one.