Time is not optional in DataPoints

[fraillt]

Changes

Make start_time and time non optional in DataPoint<T> (used for Sum and Gauge).

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 63.15789% with 14 lines in your changes missing coverage. Please review.

Project coverage is 79.5%. Comparing base (6b71301) to head (127164f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	10 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2367   +/-   ##
=====================================
  Coverage   79.5%   79.5%           
=====================================
  Files        123     123           
  Lines      21492   21488    -4     
=====================================
  Hits       17096   17096           
+ Misses      4396    4392    -4     

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[cijothomas]

It'd be good to mention in changelog as this is breaking anyone writing own exporters. I expect a lot more changes in this area, so it is probably better to club them all into one changelog entry.

[utpilla]

Start time is marked optional by spec.

@cijothomas Currently, the SDK would always populate it, but do you think there could be a scenario where we wouldn't populate the start time. With MetricProducer or something else?

[cijothomas]

Yes, producer might not provide it. so keeping Option for startTime might be safer then?

[utpilla]

Yes.

[cijothomas]

@fraillt Could you keep Option<startTime> ? Sorry I merged this already, so needs to be a separate PR.

[fraillt]

I was reading https://opentelemetry.io/docs/specs/otel/metrics/data-model/ and got impression that these fields can be in fact not optional...
Basically everything is required, except for Gauge one field:

(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.

In the proto definition same field has comment (emphasis is mine):

// StartTimeUnixNano is optional but strongly encouraged, see the detailed comments above Metric.

Maybe I'm reading outdated/invalid documentation?

[cijothomas]

(optional) A timestamp (start_time_unix_nano) which best represents the firs

^ This part is where startTime is mentioned to be optional. An external system interfacing via Producer to OTel may not have startTime.

[fraillt]

Just want to understand better... at the moment it's not possible that Gauge will have None using SdkMeterProvider.
So basically, if you want to support Gauge with optional start time, at minimum you would have to implement your own, MeterProvider, InstrumentProvider, and your own SyncInstrument. Which also means that you need to implement your own collect implementation as well.
I understand that this is part of api and user should be able to do so. I'm wondering if you know a single example where anyone actually does that ? (implement core functionality itself' instead of using SdkMeterProvider).
Or in other words, do you have any example that SdkMeterProvider is not suitable and users write their own implementation of metrics ? (just curious)

[cijothomas]

When we start supporting Producers concept, where users aggregate metrics elsewhere and bridge it to OTel, startTime can be missing.
The current sdk has no scenario for startTime being optional, but we just want to make sure our public API remains stable even when adding Producer support.

[cijothomas]

LGTM!