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MonotonicCoarse provides a coarse monotonic time. On some platforms, it is implemented in assembly, which lets us do much less work than time.Now, which gets a high precision monotonic time and a high precision wall time. The assembly code is tied to a particular Go release because it reaches into the Go internals in order to switch to the system stack for the vdso call. On my darwin/arm64 machine, there is no perf difference. On my linux/amd64 machine, MonotonicCoarse is 5x faster (50ns -> 10ns). On my linux/arm64 VM, MonotonicCoarse is 16x faster (64ns -> 4ns). We could also use this in the rate limiter and magicsock, which are two other uses of time.Now that show up in the CPU pprof when doing throughput benchmarking. Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>