Crash with "unreachable code was reached" when elapsed time is negative in metrics tracking

## Summary vcpkg crashes intermittently with "unreachable code was reached" when `track_elapsed_us()` receives a negative elapsed time value. This happens in containerized/VM environments where `std::chrono::high_resolution_clock` can go backwards. ## Error Message ``` Elapsed time to handle boost-utility:x64-linux: -1.29e+08 ns vcpkg internal error: unreachable code was reached ... ``` ## Root Cause In `src/vcpkg/metrics.cpp`, the `track_elapsed_us()` function has this assertion: ```cpp void MetricsSubmission::track_elapsed_us(double value) { if (!isfinite(value) || value <= 0.0) { Checks::unreachable(VCPKG_LINE_INFO); // Crashes here } elapsed_us = value; } ``` The problem is that `std::chrono::high_resolution_clock` is **not guaranteed to be monotonic**. In VMs, containers, or when NTP adjusts time, the clock can go backwards, producing negative elapsed time. ## Environment - OS: CBL-Mariner 2.0 (Linux container) (Also happens in Azure Linux 3.0) - Context: CI/CD pipeline (Azure DevOps) - vcpkg version: latest (cloned from main) ## Workaround ~~Disable metrics with `-disableMetrics` flag or `VCPKG_DISABLE_METRICS=1` environment variable.~~ Disabling metrics still invokes `track_elapsed_us`. ## Suggested Fix Either: 1. Use `std::chrono::steady_clock` instead of `high_resolution_clock` (guaranteed monotonic) 2. Clamp negative elapsed times to 0 or a small positive value instead of crashing 3. Skip metrics submission when elapsed time is invalid Example fix for option 3: ```cpp void MetricsSubmission::track_elapsed_us(double value) { if (!isfinite(value) || value <= 0.0) { return; // Skip invalid measurements instead of crashing } elapsed_us = value; } ```