Document how to collect event traces

Collecting from the two EventSource providers differs by tooling, not by
platform: dotnet-trace/EventPipe works the same everywhere, with ETW/PerfView
as the Windows-only alternative. The doc lists the keyword masks and levels
so sessions can enable only the areas of interest.

Assisted-by: Claude:claude-fable-5:Claude Code

doc/CollectingEventTraces.md

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+# Collecting event traces
+
+ILSpy emits performance trace events through two `EventSource` providers:
+
+| Provider | Instruments |
+| --- | --- |
+| `ICSharpCode.Decompiler` | Decompilation pipeline: per-type/per-member decompilation, type system init, assembly resolution, whole-project decompilation, per-transform timing |
+| `ICSharpCode.ILSpyX` | Frontend support: assembly loading, reference resolution, search, analyzers, bundle/zip extraction, PDB loading |
+
+Events are Start/Stop pairs, so trace viewers derive durations and nesting from the
+event timestamps. Tracing is off by default and costs nothing until a session attaches.
+
+## Keywords and levels
+
+Enable only what you need; the keyword masks combine with bitwise OR.
+
+`ICSharpCode.Decompiler` (all = `0x1F`):
+
+| Keyword | Mask | Level |
+| --- | --- | --- |
+| Decompilation (per type/member) | `0x1` | Informational (4) |
+| TypeSystem | `0x2` | Informational (4) |
+| AssemblyResolver | `0x4` | Informational (4) |
+| ProjectDecompiler | `0x8` | Informational (4) |
+| Transforms (per IL/AST transform, high volume) | `0x10` | Verbose (5) |
+
+`ICSharpCode.ILSpyX` (all = `0x3F`):
+
+| Keyword | Mask | Level |
+| --- | --- | --- |
+| AssemblyLoad | `0x1` | Informational (4) |
+| Resolver | `0x2` | Informational (4) |
+| Search | `0x4` | Informational (4) |
+| Analyzers | `0x8` | Informational (4) |
+| Packages (per-entry extraction is Verbose) | `0x10` | Informational (4) / Verbose (5) |
+| DebugInfo | `0x20` | Informational (4) |
+
+Provider specs below use the `Name:KeywordMask:Level` syntax.
+
+## All platforms: dotnet-trace (EventPipe)
+
+Works identically on Windows, Linux and macOS.
+
+```
+dotnet tool install --global dotnet-trace
+```
+
+Attach to a running ILSpy instance (start ILSpy first, interact while collecting,
+Ctrl+C to stop):
+
+```
+dotnet-trace ps
+dotnet-trace collect -p <pid> --providers "ICSharpCode.Decompiler:0x1F:5,ICSharpCode.ILSpyX:0x3F:5"
+```
+
+Or launch ILSpy under the collector to also capture startup (assembly list loading):
+
+```
+dotnet-trace collect --providers "ICSharpCode.Decompiler:0x1F:5,ICSharpCode.ILSpyX:0x3F:5" -- <path-to>/ILSpy
+```
+
+Viewing the resulting `.nettrace` file:
+
+- Windows: open it in PerfView (Events view) or Visual Studio.
+- Cross-platform: `dotnet-trace convert --format speedscope trace.nettrace` and open
+  the output at <https://speedscope.app>, or `--format chromium` for `about:tracing`
+  in a Chromium browser.
+
+## Windows alternative: ETW
+
+PerfView (`choco install perfview`) collects the same providers over ETW, which also
+gives you the Start/Stop duration pairing in the Events view:
+
+```
+PerfView "/onlyProviders=*ICSharpCode.Decompiler,*ICSharpCode.ILSpyX" run ILSpy.exe
+```
+
+Other options:
+
+- `PerfView collect "/onlyProviders=..."` to attach machine-wide instead of launching.
+- `wpr`/Windows Performance Analyzer with a custom profile listing the two providers.