Metrics: Add support for exemplars

[codeboten]

As per the SDK specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar

[paprickar]

Hey @codeboten,

do you might have an update on this?

[codeboten]

@paprickar i do not. Maybe one of the @open-telemetry/python-maintainers can chime in with more info

[lzchen]

@paprickar
We haven't started working on exemplars yet.

[paigerduty]

Curious about the status of this work - planned, in progress or perhaps needs a contributor to pick up?

[lzchen]

@paigerduty

Not planned nor in progress. We welcome any contributors to pick this up!

Also, specs are merged for exemplars! open-telemetry/opentelemetry-specification#3870

[lzchen]

open-telemetry/opentelemetry-specification#4034

[fcollonval]

@lzchen I'm interesting in helping moving forward with this. Could you provide some pointers? Is the early attempt #936 a good starting point?