feat: support 'otel:bundle:load' diagnostics_channel message for instrumentation in bundles

[trentm]

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(This is the core-repo part of a proposed diagnostics_channel-based approach to supporting instrumentation in bundles. There will be a coming contrib-repo PR to add an esbuild-plugin package. More details coming.)

checklist

• [ ] add changelog message (I'm skipping for now to avoid merge conflicts while this is still draft)