Control time unit of latency measurements

[guydc]

Version

1.11.x (beta)

Is your feature request related to a problem? Please describe.

Envoy measurements are provided in truncated milliseconds. For example, the value of the x-envoy-upstream-service-time is computed in counted milliseconds: https://github.com/envoyproxy/envoy/blob/ff24017783df8cb0f342d7532b565bd0cd54f106/source/common/router/router.cc#L1448. It should be possible to produce more accurate metrics in microseconds, for the various latency measurements (duration, service time, etc.).

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Describe alternatives you've considered

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Additional Context

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