Can Stdout exporter write timestamp in human readable format?

[cijothomas]

Given stdout exporters are meant for human consumption (learning purposes, not production use), I think the timestamps should be written in human readable format as opposed to ticks_since_epoch (eg: https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-stdout/src/metrics/transform.rs#L274).

I can send a short PR to make this happen for one signal, and if no objections, apply to all signals.

[jtescher]

Resolved in #1192