RPC: continuous profiling for Python/JS/Go subprocesses

[jkschneider]

Summary

Each language's RPC server now starts the Pyroscope SDK when PYROSCOPE_SERVER_ADDRESS is set in the environment, with PYROSCOPE_TAGS forwarded verbatim and a runtime=python|node|go tag added so flame graphs in a shared modcli Pyroscope application can be sliced by which subprocess produced them.

The env-var propagation chain — recipe-worker JVM → modw → mod CLI JVM → ProcessBuilder for the RPC subprocess — works without code changes here because RewriteRpcProcess.run() calls pb.environment().putAll(extras) rather than clearing inherited env. So once the saas-side CliProcessEnvContributor (which today wires the JVM Pyroscope agent on the mod CLI) is extended to also set PYROSCOPE_SERVER_ADDRESS / PYROSCOPE_TAGS / PYROSCOPE_APPLICATION_NAME, all four runtimes profile under the same modcli app, separated by the runtime tag.

Per-language notes

• Python (rewrite-python/rewrite/src/rewrite/rpc/server.py): _init_pyroscope() called from main() after argparse. pyroscope-io is a new optional dep ([profiling] extra) so local dev and CI without the package don't break — the function warns and no-ops on ImportError.
• JS (rewrite-javascript/rewrite/src/rpc/server.ts): initPyroscope(logger) called from main() right after the logger is constructed. @pyroscope/nodejs is a new optionalDependencies entry so npm install doesn't fail when the binding's native components aren't available; initPyroscope catches the require and warns.
• Go (rewrite-go/cmd/rpc/main.go): initPyroscope() is the first line of main(). github.com/grafana/pyroscope-go v1.2.8 is a regular dep — Go has no optional-dep concept and the SDK is small and pure-Go (uses stdlib pprof under the hood). The function short-circuits on missing env so non-profiled deployments pay one os.Getenv at startup.
• C#: no change needed in rewrite-csharp. CoreCLR loads the Pyroscope native profiler purely from CORECLR_ENABLE_PROFILING / CORECLR_PROFILER / CORECLR_PROFILER_PATH, which the saas-side contributor will set alongside PYROSCOPE_*.

Test plan

• python -c "import ast; ast.parse(open('.../server.py').read())" — parses
• tomli parse of pyproject.toml — parses; [profiling] extra resolves
• npx tsc --noEmit -p tsconfig.test.json (rewrite-javascript) — clean
• go build ./cmd/rpc (rewrite-go) — clean
• go vet ./cmd/rpc (rewrite-go) — clean
• End-to-end on dev recipe-worker: with profiling capability + flag enabled, trigger a Python recipe run and confirm application=modcli flame graphs in Grafana split by runtime tag (python, node, go, jvm). Requires the saas-side CliProcessEnvContributor extension to land first.
• Confirm [profiling] extra installs cleanly in mod-cli-base's Python venv.
• Confirm optionalDependencies doesn't fail npm install on a clean container without the native binding present.