[Feature Request]: Add User Worker Threads info to Dataflow backend

[EllaDing]

What would you like to happen?

Reporting some signals from legacy user workers to Dataflow backend for internal autotuning policies to use. The signals planned to be plumbed:

1. number of active threads
2. outstanding bytes & its limit
3. outstanding number of bundles & its limit

Issue Priority

Priority: 3 (nice-to-have improvement)

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[EllaDing]

.take-issue