## Why
Turn completion metrics used session-level telemetry, which could label usage with the wrong model after a model switch. A turn can also include compaction and responses from different models, so a single token total cannot attribute usage correctly.
## What changed
- Accumulate token usage by model using each response's resolved step settings, and emit one histogram sample per model and token type per turn.
- Preserve zero-valued samples when no usage is reported, without adding a sample for an unused selected model when only compaction ran.
- Use turn telemetry for tool-call, memory, network-proxy, and running-process metrics.
## Testing
Add regression tests for model switches, step-level model changes, compaction followed by a response or an aborted turn, and turns without responses. Verify repeated requests aggregate into one sample per model and earlier session usage is excluded.
GitOrigin-RevId: 03ab1dcde7da0b16b3a500a9a05e4c86583ee585
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