sdk/metric: Set StartTime for gauge metric points

[pellared]

Problem Statement

From specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#gauge

(optional) A timestamp (start_time_unix_nano) which best represents the first possible moment a measurement could be recorded. This is commonly set to the timestamp when a metric collection system started.

Proposed Solution

I am not sure what "metric collection system started" exactly means.

We could capture time.Now() when either:

• meter provider is created
• instrument is created
• first collect occurs
• startup with an init function

Additional Context

Found it missing when debugging #4268

[MrAlias]

It could also mean we get the current time on startup with an init function.

Not sure what the appropriate timestamp is though.

[pellared]

I created open-telemetry/opentelemetry-specification#3576

[pellared]

I think right now we can set it to the time when metric provider is created until it is better specified.

[MrAlias]

This apears to not need a fix for the SDK GA. Can we remove it from the project?

It might also make sense to hold off on a solution until there is consensus in the spec so we don't have revert things.

[pellared]

This apears to not need a fix for the SDK GA. Can we remove it from the project?

It might also make sense to hold off on a solution until there is consensus in the spec so we don't have revert things.

Moved to Go: Metric SDK (Post-GA)