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

### 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)

### Issue Components

- [ ] Component: Python SDK
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- [ ] Component: Typescript SDK
- [ ] Component: IO connector
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- [X] Component: Google Cloud Dataflow Runner