Different unit of timestamp in metrics_service in linux and macos

[lomik]

Description:
Envoy sends a different value for timestamps to metrics_service in linux and macos: nanos in linux and micros in macos.
Problem reproduced in 1.7.0 and in current master.

Repro steps:
Sample "metrics_service" and envoy config are here: https://github.com/lomik/envoy-issue/tree/master/metric_service_timestamp

Macos output:

&io_prometheus_client.Metric{Label:[]*io_prometheus_client.LabelPair(nil), Gauge:(*io_prometheus_client.Gauge)(nil), Counter:(*io_prometheus_client.Counter)(nil), Summary:(*io_prometheus_client.Summary)(0xc0001d61b0), Untyped:(*io_prometheus_client.Untyped)(nil), Histogram:(*io_prometheus_client.Histogram)(nil), TimestampMs:1535464990220755}

# TimestampMs:1535464990220755

Linux output:

&io_prometheus_client.Metric{Label:[]*io_prometheus_client.LabelPair(nil), Gauge:(*io_prometheus_client.Gauge)(0xc420026818), Counter:(*io_prometheus_client.Counter)(nil), Summary:(*io_prometheus_client.Summary)(nil), Untyped:(*io_prometheus_client.Untyped)(nil), Histogram:(*io_prometheus_client.Histogram)(nil), TimestampMs:1535465054722145763}

# TimestampMs:1535465054722145763

[lizan]

@jmarantz Can you take a look on this? cc @zuercher for macOS.

[jmarantz]

This is already being addressed in #4288 ?

[zyfjeff]

Sorry, I didn't explain it in the issue in time, I am already dealing with this problem.

[jmarantz]

agreed; please assign to @zyfjeff

[zuercher]

I think assignees need to be members of the org. Leaving it unassigned for now.