## Why
The existing request duration metric starts at dispatch, leaving out time spent waiting before dispatch.
## What changed
Add `exec_server_request_total_duration_seconds` to measure time from decoded receipt until response enqueue or disconnection, including queueing. Record it alongside the existing duration metric with the same `method` and `result` labels for success, error, and disconnection paths. Preserve existing dispatch and queue timing semantics.
## Testing
Add coverage for successful requests, route errors, unknown methods, response delivery failures, and disconnection during execution. Extend admission-wait coverage to verify total timing includes queueing and route setup without double-counting, and assert each completion and duration is recorded once.
GitOrigin-RevId: ed0f719751b821b8f9f9556914b0f67dc6c5ad8f
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