Add support for PromQL info function

[puckpuck]

Add support for the PromQL info() function which joins metrics with target_info. Currently this function is experimental with Prometheus, and does require changes to many popular collector receivers for infra metrics to adopt properly. When those changes take place across the Otel community, and the function is promoted within Prometheus we should ensure the demo is configured to support the new recommended model.

We started this effort with PR #2869 but decided that we are a little early for the transition, and we should revisit in 2-3 months.

[aknuds1]

Currently this function is experimental with Prometheus, and does require changes to many popular collector receivers for infra metrics to adopt properly

The PromQL info function doesn't require changes to the receivers. Instead, Prometheus itself requires for OTel resources to have the service.instance.id and/or service.name attributes, in order to be able to ingest resource attributes (into target_info). This is per the OTLP metric points to Prometheus spec. The info function merely gets labels corresponding to resource attributes from target_info.

Prior to the introduction of the info function, one would enrich query results by joining with the target_info metric. For the full picture, see our blog post on the info function.

On the topic of info getting promoted through Prometheus; even though it is still experimental, due to technical challenges, it is already our recommendation.

[aknuds1]

@lmolkova Could you perhaps comment on this issue, and on whether it's really a problem for the OTel Demo to depend on injecting service.instance.id through processors where necessary?

[puckpuck]

If Prometheus requires service.instance.id and/or service.name to be present, this will require Otel collector receivers for infra metrics to add these resource attributes. We should not need to resort to using transform processor rules to support this for the variety of different infra metrics receivers.