feat: metrics sdk support for schema_url and attributes

[xuan-cao-swi]

Descriptions

Closes #1708
Closes #1709

[kaylareopelle]

I like that this adds the plumbing needed to attach a schema URL and attributes to a meter.

However, I'm concerned about adding the schema URL at this time. I don't think we have the semantic conventions setup to support schemas. I'm not very versed in this OTel concept, so I might be misunderstanding how it's supposed to work.

The telemetry schemas spec says there are three parties coupled by the schema: OTel semantic conventions, telemetry sources and telemetry consumers. The presence of a schema URL may benefit a telemetry consumer (AKA a vendor backend that uses this value to deliver a customer experience), but we're missing the features needed to link a schema URL with semantic conventions emitted by our telemetry sources, which seems like a large gap to me.

In #1651, we're looking at updating the conventions that can be emitted, but I don't believe there is any link to schema URL to determine what conventions are emitted.

@robbkidd, I feel like you have the best grasp on semantic conventions here, and I hope that extends to schema URLs too. What are your thoughts on the SDK's readiness to add support for schema URLs? If we're not ready, what would we need to accomplish to become ready?

[xuan-cao-swi]

... but we're missing the features needed to link a schema URL with semantic conventions emitted by our telemetry sources, which seems like a large gap to me.

Totally agree. I was thinking just put there as a placeholder for spec compliance.