diagnostics_channel: return original thenable #5183

Job	Run time
Build slim tarball	1m 57s
x86_64-darwin: with shared libraries	29m 47s
aarch64-linux: with shared libraries	18m 3s
x86_64-linux: with shared libraries	21m 26s
aarch64-darwin: with shared libraries	22m 17s
	1h 33m 30s

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